UNIVERSITY    OF   CALIFORNIA 


DEPARTMENT  OF  EDUCATION 


Gift  of 


Received 


\ 


ASTRONOMY 


FOR 


SCHOOLS  AND  GENERAL  READERS 


BY 

ISAAC  SHARPLESS,  Sc.D., 

PRESIDENT  OF  HAVERFORD  COLLEGE, 

AND 

GEO.  MORRIS   PHILIPS,  PH.D., 

PRINCIPAL  OP  STATE   NORMAL   SCHOOL,   WEST  CHESTER,   PA 


FO  UBTH  EDITION— RE  VISED. 


PHILADELPHIA: 

J.  B.  LIPPINCOTT   COMPANY 


!>3» 

--.'..*       -ceo*, 


Copyright,  1832,  by  J.  B.  LIPPINCOTT  A  Co. 


Copyright,  1892,  by  J.  B.  LIPPINCOTT  COMPANY. 


EDUCATION  DEPT, 


PREFACE. 


ASTKONOMY  is  not  studied  in  the  lower  and  inter- 
mediate schools  of  the  United  States  as  much  as  its 
importance  and  interest  demand.  Its  phenomena  are 
so  striking,  so  well  calculated  to  awaken  thought,  and 
so  much  objects  of  common  notice,  that  an  intelligent 
appreciation  of  their  causes  and  relations  is  greatly  to 
be  desired. 

This  book  is  believed  to  be  written  so  that  any 
person  of  ordinary  education  and  intelligence  can  un- 
derstand it.  £To  knowledge  of  mathematics  beyond 
arithmetic  is  necessary,  except  that  in  a  few  cases  trig- 
onometrical solutions  of  important  problems  have  been 
given  in  foot-notes  for  the  benefit  of  those  who  un- 
derstand such  methods.  Special  effort  has  been  made 
to  render  clear  the  abstruse  points  in  the  science, — 
with  what  success  can  be  judged  from  the  explanations 
of  the  Transit  of  Venus,  the  Precession  of  the  Equi- 
noxes, the  Tides,  etc.  Particular  care  has  been  taken 
to  distinguish  between  theories  and  established  facts, 
even  when  the  former  seem  to  be  highly  probable; 
while  mere  speculations  are  altogether  excluded.  The 

illustrations  have   been   carefully  chosen.      They  are 

3 

60SG66 


4  PREFACE. 


believed  to  be  better  and  more  numerous  than  are  usu- 
ally found  in  books  of  this  character,  and  it  is  hoped 
that  they  will  render  considerable  help  in  making  the 
subject  clear  and  interesting. 

The  most  original  feature  of  the  work  is  the  direc- 
tion everywhere  given  for  observations  with  the  naked 
eye  and  with  small  telescopes.  As  illustrations  of  this 
may  be  mentioned  the  methods  of  observing  meteors, 
variable  stars,  and  the  phenomena  of  Jupiter's  satellites. 
This  plan  of  setting  students  at  practical  work  has  been 
so  successful  in  chemistry,  botany,  and  other  sciences, 
that  it  seems  to  be  quite  time  to  use  it  in  astronomy. 
It  may  be  that  many  of  the  readers  of  this  little  book 
will  be  surprised  at  the  large  amount  of  interesting  and 
valuable  observation  that  can  be  made  with,  the  aid  of 
a  very  small  glass,  and  even  with  the  unassisted  eye. 


PREFACE  TO  THE  FOURTH  EDITION, 


THE  present  edition  has  been  carefully  revised.  The 
recent  discoveries  of  importance  have  been  included,  as 
well  as  the  best  of  new  theories.  The  authors  thank- 
fully acknowledge  the  receipt  of  valuable  suggestions 
from  teachers  as  to  better  methods  of  presenting  certain 
portions  of  the  subject,  which  they  have  freely  used. 
The  book  is  believed  to  be  not  only  adapted  to  class 
use,  but  reliable  and  thoroughly  modern. 


CONTENTS, 


INTEODUCT1ON. 

MM 

History  of  Astronomy .        9 

General  View  of  the  Heavens 18 

Usefulness  of  Astronomy 26 


PAET  I. 

THE    SOLAR   SYSTEM. 

CHAPTER  I.— General  View  of  the  Solar  System        .        .        .  28 

II.— The  Sun         .        . 44 

III.— The  Inferior  Planets ' 65 

Mercury •,-'.,        .66 

Venus         .        .    .    .        .        .                         .  70 

IV.— The  Earth 79 

The  Tides .        .  118 

V.— The  Moon 124 

VI.— Eclipses 144 

VII.— The  Superior  Planets 153 

Mars 153 

The  Minor  Planets 159 

Jupiter 163 

Saturn 173 

Uranus 181 

Neptune .        .  183 

VIII.— Comets  and  Meteors .189 

Comets 189 

Meteors 204 

Relation  between  Comets  and  Meteors      .        .214 
1*  •  6 


6  CONTENTS. 


PAKT   II. 

THE   SIDEREAL    SYSTEM. 

jPAGE 

CHAPTER  I — The  Constellations         .        .        .        «...  217 

Description  of  the  Constellations      .        .        .  230 

II.— Double  Stars          .        .        .                 .        .        .  249 

Variable  and  New  Stars 253 

Clusters  and  Nebulas     .        .        .        ,    __.  .        .  260 

Structure  of  the  Universe             .                         .  272 


PAET   III. 

PROPERTIES  OF  LIGHT,  AND  ASTRONOMICAL  INSTRUMENTS  279 

APPENDICES. 

I. — LIST  OP  LARGE  TELESCOPES 305 

II. — ASTRONOMICAL  SYMBOLS       .        .        .  306 

III. — LENGTHS  OF  DAYS,  MONTHS,  AND  YEARS  .        .        ,  307 

IV. — STATISTICS  OF  PLANETS,  SUN,  AND  MOON  .    ' "  .  -     .  308 

V.— PERIODIC  COMETS  .        .        .        .        ,        .        %   .  •  .  309 

VI.— LIST  OF  NOTED  DOUBLE  STARS  .        .        .        .         .  310 
711. — LIST  OF  NOTED  CLUSTERS  AND  NEBULA  ,        .        .311 


SUGGESTIONS  TO  TEACHERS. 


Aids. — A  celestial  globe,  twelve  inches,  or  there- 
abouts, in  diameter,  is  most  useful  in  illustrating 
and  explaining  many  astronomical  phenomena,  and  in 
finding  the  constellations  and  principal  stars.  Be 
sure  that  the  globe  has  a  horizontal  ring  about  the 
middle.  A  Planisphere  is  a  tolerable  substitute  for  a 
globe,  and  much  cheaper.  A  Star  Lantern  is  also  very 
convenient.  A  good  star-map  is  important.  A  tele 
scope  of  any  size,  or  even  a  good  spy-glass  or  pair  of 
opera-glasses,  will  add  much  interest  to  the  study. 

Methods  of  Instruction. — Each  teacher  has  his  own 
method  of  conducting  recitations,  but  the  authors'  ex- 
perience leads  them  to  prefer  the  topical  method,  and 
whenever  possible  they  would  have  the  student  learn 
the  topics  in  their  order,  so  as  to  get  a  complete  and 
connected  knowledge  of  the  subject  The  headings  of 
the  paragraphs  and  the  arrangement  of  the  topics  will 
facilitate  this.  Reviews  here,  as  elsewhere,  will  be 
found  to  be  very  valuable.  Distances,  dimensions,  etc., 
as  given  in  round  numbers,  should  be  learned  and 
made  perfectly  familiar  by  frequent  repetition.  Par- 
ticular attention  ought  to  be  paid  to  the  questions  and 


g<    :  &Tr&®£80N8  TO   TEACHERS. 

suggested  problems  in  the  foot-notes.  This  will  test 
the  pupil's  knowledge  and  make  it  more  thorough. 
A  teacher  should  never  be  content  until  his  class  un- 
derstands each  point  thoroughly ;  and  it  must  not  be 
forgotten  that  no  one  can  explain  clearly  to  a  class 
what  he  does  not  clearly  understand  himself.  It  is 
hoped  that  every  teacher  who  essays  to  teach  this 
subject  will  acquaint  himself  thoroughly  with  it  by 
making  use  of  standard  works  upon  astronomy ;  and 
he  may  rest  assured  that  no  knowledge  that  he  can 
acquire  will  be  more  interesting,  or  more  valuable 
everywhere  and  anywhere,  than  this. 

Practical  Work. —  Above  all  things,  the  teacher 
must  not  neglect  the  practical  work.  Let  him  take 
his  class  out  under  the  clear  sky  and  point  out  the 
constellations,  principal  stars,  and  planets.  Let  him 
make  himself,  and  lead  his  students  to  make,  the  ob- 
servations described  in  the  following  pages.  He  will 
be  surprised  at  the  interest  awakened,  and  at  the  valu- 
able results.  A  common  household  almanac  will  be 
of  great  aid  here.  There  is  much  more  in  an  almanac 
than  most  people  see 


INTRODUCTION. 


History  of  Astronomy. 

1.  Early  History.  —Astronomy,  the  science  of  the 
heavenly  bodies,  is  probably  the  oldest  of  all  the  sci- 
ences. So  old  is  it  that  there  is  no  trustworthy  account 
of  its  origin;  indeed,  almost  every  famous  nation  of 
antiquity  claimed  the  honor  of  originating  it.  Nor  is 
it  hard  to  see  why  this  science  should  have  been  culti- 
vated so  early.  The  first  men  had  no  books  to  occupy 
their  time,  hence  they  observed  nature.  The  most 
striking  occurrence  was  the  succession  of  day  and 
night,  the  one  lighted  up  by  the  brilliant  sun,  the  other 
dark,  or  feebly  illuminated  by  the  wonderful  stars  and 
the  curiously  changing  moon.  These  changes  were  a 
very  natural  division  of  time,  the  only  ones  they  had. 
As  men  knew  not  the  true  God,  they  naturally  turned 
to  the  heavens  for  objects  of  worship,  and  this  led  to 
careful  study  and  observation  of  the  heavenly  bodies 
by  priests  and  other  ministers  of  religion.  Besides, 
the  occupations  and  modes  of  living  of  our  earliest  an- 
cestors were  most  favorable  to  the  study  of  astronomy. 
As  hunters,  shepherds,  and  farmers,  their  lives  were 
snent  in  the  open  air,  by  night  as  well  as  by  day.  In 
travelling  over  the  thinly-peopled  earth,  and  the  sea  as 

9 


S;  >i *.-  *t i ...; ^ INTRODUCTION. 


well,  the  stars  were  their  guides.  It  is  not  surprising, 
then,  that  these  men,  with  no  instruments,  no  books, 
no  schools,  knew  much  about  astronomy.  They  seem, 
in  fact,  to  have  known  more  about  the  appearance  and 
phenomena  of  the  heavens  than  we  generally  do. 

2.  Astronomy  of   the  Chaldeans. — According  to  the 
Greek  historians,  the  Chaldeans  were  the  first  astrono- 
mers.    These  people  lived  along  the  Euphrates  River 
in  Asia,  in  and  about  the  city  of  Babylon.     They  kept 
careful  records  of  the  movements  and  phenomena  of 
the  heavenly  bodies.    By  these  records  they  discovered 
that  the  eclipses  of  the  sun  and  moon  are  almost  ex- 
actly repeated  every  eighteen  years,  and  thus  success- 
fully predicted  eclipses.     But  of  the  real  causes  of 
eclipses,  or  of  the  nature,  distance,  or  real  motions  of 
the  heavenly  bodies,  these  ancient  astronomers  knew 
nothing. 

3.  Astronomy  among  other  Ancient  Nations. — The  Egyp- 
tians, like  the   Chaldeans,  studied  astronomy  in  very 
ancient  times.    Some  writers  contend  that  their  famous 
pyramids  are  so  built  as  to  show  great  astronomical 
knowledge,  but  very  little  is  certainly  known  about 
this  matter,  or  about  their  advancement  in  astronomy. 
It  seems  to  be  proved  that  the  Chinese  had  a  knowl- 
edge of  astronomy  very  early, — more  than  four  thou- 
sand years  ago,  according  to  their  own  claim ;  but  the 
evidence  of  this  extreme  age  of  the  science  among  them 
is  doubtful.     Their  records  relate  that  about  that  date 
Ho  and  Hi  were  the  two  royal  astronomers,  whose  duty 
it  was  to  predict  all  eclipses,  but  that,  giving  themselves 
up  to  the  pursuit  of  pleasure,  they  neglected  their  du- 
ties, and  an  eclipse  of  the  sun  occurred  without  being 
predicted.     The  whole  nation  was  thus  exposed  to  the 


INTRODUCTION.  \\ 


anger  of  their  gods,  because  of  the  omission  of  the 
religious  ceremonies  always  performed  upon  such  oc- 
casions. The  unfortunate  astronomers  were  imme- 
diately put  to  death.  It  is  certain,  however,  that  the 
Chinese  made  reliable  astronomical  observations,  some 
of  which  are  of  use  to  us,  at  least  two  thousand  five 
hundred  years  ago. 

The  Hindoos  also  claim  to  have  been  the  first  to 
study  astronomy.  They  have  proved  their  claim  to  an 
extensive  knowledge  of  the  subject,  but  whether  they 
borrowed  this  knowledge  from  the  neighboring  nations, 
or  gained  it  by  observation,  is  uncertain. 

4.  Greek  Astronomy. — Astronomy  was  a  favorite  sci- 
ence with  the  ancient  Greeks.  But,  as  was  the  case 
with  a  great  part  of  their  science,  their  astronomy  was 
imagined  rather  than  observed.  Some  of  their  astron- 
omers advanced  surprisingly  correct  theories  of  the 
heavenly  bodies,  but  seem  to  have  made  little  effort 
to  prove  them.  Certain  of  their  earliest  philosophers 
taught  that  the  earth  is  a  sphere,  a  belief  not  original 
with  Columbus,  as  some  people  think,  but  one  taught 
in  Greece  two  thousand  years  before  Columbus  was 
born.  Later  some  of  the  Greeks  taught  that  the  sun 
is  the  centre  of  the  system  of  planets  to  which  the  earth 
belongs,  and  that  all  revolve  about  the  sun;  others 
taught  that  day  and  night  are  caused  by  the  revolution 
of  the  earth  upon  its  axis.  These  great  truths  are  the 
foundation  of  modern  astronomy,  but  the  Greek  phi- 
losophers brought  forth  so  little  evidence  in  support  of 
these  guesses,  and  mingled  so  many  absurdities  with 
them,  that  they  were  not  generally  believed,  and  were 
soon  forgotten.  Notwithstanding  their  general  habit 
of  neglecting  experiments  for  theories,  the  Greeks 


12  INTRODUCTION. 


achieved  some  substantial  results.  They  made  obser- 
vations which  were  of  use  to  succeeding  astronomers, 
they  greatly  improved  the  reckoning  of  time,  and  de- 
termined the  length  of  the  year  to  be  three  hundred 
and  sixty-five  and  one-fourth  days,  which  is  wonder- 
fully near  its  exact  length.1 

5.  The  Alexandrians. — For  a  few  hundred  years  be- 
fore and  after  the  Christian  era,2  the  city  of  Alexandria 
in  Egypt  was  famous  for  its  learning.  Its  astronomers 
were  the  most  skilful  that  had  yet  lived.  They  at- 
tempted to  find  the  relative  distances  of  the  sun  and 
moon  from  the  earth.  The  method  employed  was  a 
correct  and  very  ingenious  one,  but  from  the  imperfec- 
tions of  their  observations  their  results  were  far  from 
the  truth.  They  determined  the  width  of  the  torrid 
zone  with  great  exactness,  and  found  the  circumference 
of  the  earth  with  surprising  accuracy,  using  the  method 

1  The  ancients  found  the  length  of  the  year  by  means  of  a  gnomon 
(no'mon).  This  was  a  pillar  set  up  to  cast  a  shadow,  which  was 
measured  at  noon  every  day.  When  the  noonday  sun  was  lowest 
down  in  the  sky  the  shadow  of  the  gnomon  was  longest,  as  a  little  re- 
flection will  show.  This  time  of  year  is  called  the  winter  solstice, 
and  marks  the  time  when  the  sun  is  farthest  south  of  the  equator  and 
is  shining  directly  down  upon  the  tropic  of  Capricorn ;  according  to 
our  reckoning,  this  is  about  the  21st  of  December.  After  that  date 
the  noonday  shadow  grows  shorter,  because  the  sun  gets  farther  north 
every  day.  Now,  if  the  day  upon  which  the  gnomon's  shadow  is 
shortest  is  found,  and  the  days  are  carefully  counted  until  the  short- 
est shadow  comes  again,  the  length  of  the  year  is  found.  It  is  inter- 
esting to  know  that  the  obelisks  of  Egypt,  one  of  which  has  lately 
been  brought  to  New  York  and  set  up  in  the  Park  there,  are  thought 
to  have  been  used  as  gnomons. 

If  the  gnomon  were  south  of  the  equator,  would  it  make  any  change 
in  this  explanation  ?  Could  the  time  of  noon  be  found  by  measuring 
the  length  of  the  shadow  ? 

*  What  is  meant  by  this  ? 


INTRODUCTION.  13 


which  is  still  used  as  the  very  best  one  known.  It 
will  be  described  farther  on  in  the  book.  Euclid,1  who 
gave  us  the  geometry  which  in  substance  is  still  uni- 
versally used,  lived  in  Alexandria  during  this  period, 
and  contributed  to  the  advancement  of  astronomy. 

6.  Hipparchus.2 — This  was  the  greatest  of  the  ancient 
astronomers,  and  well  deserves  his  title,  "  Father  of 
Astronomy."     He  lived  upon  the  island  of  Rhodes,  in 
the  Mediterranean  Sea,  about  150  B.C.3     Hipparchus 
determined  the  length  of  the  year  to  within  about  four 
minutes  of  its  true  length.     He  discovered  that  the 
distance  from  the  sun  to  the  earth  varies  throughout 
the  year,  and  he  made  several  most  important  discov- 
eries in  the  movements  of  the  heavenly  bodies.     He 
made  the  first  catalogue  of  the  stars,  fixing  the  position 
of  over  a  thousand  of  them.     This  catalogue  is  one  of 
the  most  valuable  possessions  of  modern  astronomy. 
Hipparchus  invented  the  science  of  trigonometry,  and 
first  used  latitude  and  longitude  to  determine  the  posi- 
tions of  places  on  the  earth. 

7.  Ptolemy4  and  his  System. — This  most  famous  astron- 
omer of  antiquity  lived  at  Alexandria  about  130  A.D.5 
He  made  few  observations  himself,  but  collected  the 
results  of  other  men's  work  and  wrote  them  down,  to- 
gether with  some  important  investigations  of  his  own, 
and  it  is  to  him  that  we  owe  almost  all  our  knowledge  of 
ancient  astronomy.     His  great  work  upon  astronomy, 
the  "  Almagest,"  still  exists,  and  for  fourteen  hundred 
years  it  was  the  highest  and  the  only  authority  upon 

1  Euclid  (yoo'klid)  flourished  about  300  B.C. 

8  Pronounced  Hip-ar'kus. 

•  What  is  meant  by  this  ?     How  long  ago  were  these  timeg  ? 

4  Pronounced  Tol'e-my. 


14  INTRODUCTION. 


the  subject.  The  foundations  of  the  Ptolemaic  system 
are  that  the  earth  is  a  sphere,  that  it  is  the  centre  of  the 
universe,  and  that  it  is  stationary,  while  all  the  heavenly 
bodies  revolve  about  it  every  twenty-four  hours.  That  the 
earth  is  a  sphere  Ptolemy  proved  by  the  fact  that  at 
places  west  of  the  observer  the  sun  rose  and  set 
later,  and  at  places  east,  earlier;1  and  also  because  as 
one  goes  north  the  pole-star  rises  higher  in  the  sky, 
while  it  sinks  lower  as  he  goes  south.  That  the  earth 
stands  still  while  the  sun  and  stars  revolve  about  it, 
Ptolemy  argued  was  simply  common  sense.  And  he 
took  some  pains  to  show  the  absurdity  of  the  belief 
that  these  phenomena  are  caused  by  the  turning  of  the 
earth  upon  its  axis.  Ptolemy's  theory  explains  the 
apparent  motions  of  the  sun,  moon,  and  stars  pretty 
well,  but  the  apparent  motions  of  the  planets2  are  so 
peculiar,  as  will  be  explained  when  these  are  treated 
of,  that  he  was  forced  to  conclude  that  these  bodies  do 
not  move  in  circles  about  the  earth,  but  in  very  com- 
plicated circular  paths,  composed  of  series  of  loops. 
This  is  the  theory  of  the  universe  which  was  accepted 
everywhere  without  question  until  the  sixteenth  cen- 
tury.3 

1  A  little  thought,  aided  perhaps  by  a  diagram,  will  make  this  rea- 
soning clear.     At  St.  Louis  the  sun  rises  and  sets  an  hour  later  than 
at  Philadelphia  ;  hence  St.  Louis  time  is  an  hour  behind  Philadelphia 
time.    How  would  this  affect  travellers  ?   How  does  it  affect  railroad- 
trains  ? 

2  A  few  of  what  are  commonly  called  stars  are  planets,  and  are 
comparatively  near  to  us.    They  resemble  the  earth  in  many  respects. 
The  others  are  properly  called  stars,  and  are  suns,  situated  at  immense 
distances  from  us.     The  word  planet  is  derived  from  a  Greek  word, 
meaning  a  wanderer,  because  these  bodies  wander  among  the  stars. 

8  The  sixteenth  century  began  at  the  beginning  of  the  year  1501 


INTRODUCTION.  15 


8.  Copernicus1  and  his  System. — It  has  already  been 
mentioned  that  some  of  the  old  Greek  astronomers 
held  and  taught  the  true  theory  of  the  heavenly  bodies, 
but,  substantiated  by  no  proofs  and  borne  down  by 
the  great  authority  of  Ptolemy,  their  teachings  had 
long  since  been  forgotten.     And  it  was  not  until  about 
1500  A.D.  that  Copernicus,  a  Prussian  mathematician 
and   astronomer,  revived   and  firmly  established  the 
essential  truths  of  astronomy.     He  showed  that  the 
earth  and  planets  revolve  about  the  sun  as  a  centre, 
and  that  the  daily  risings  and  settings  of  the  heav- 
enly bodies  are  caused  by  the  turning  of  the  earth 
upon  its  axis.    Although  his  theories  were  not  strictly 
original  with  him,  and  although  he   left  them  very 
incomplete,  yet  Copernicus  has  been  honored  greatly 
and  justly  for  bringing  forward   and   clearly   stating 
the   true  principles  of  astronomy,  at  the  same  time 
showing  good  reasons  for  his  belief;  as  well  as  for 
his  courage  in  thus  breaking  away  from  the  ignorance 
and  superstition  of  his  age.    His  work  upon  the  subject 
was  not  published  until  just  at  the  close  of  his  life,  and 
the  first  printed  copy  of  it  was  put  into  his  hands  only 
a  few  hours  before  his  death.     In  his  honor  our  theory 
of  astronomy  is  still  called  the  Copernican  System. 

9.  Kepler.2 — This   great   mathematical   astronomer 
followed   Copernicus.      His  whole   life  was  spent  in 
laborious  calculations.     His  name  is  most  frequently 
mentioned  in  connection  with  three  great  laws,  which 
explain  the  paths,  motions,  and  distances  of  the  planets. 

and  ended  at  the  close  of  the  year  1600.     What  century  is  this? 
When  did  it  begin,  and  when  will  it  close  ? 

1  Copernicus  (ko-per'ni-kus),  1473-1543. 

*  Kepler,  a  German,  1571-1630. 


16  INTRODUCTION. 


These  three  laws  (see  page  36),  which  would  scarcely 
fill  a  half  of  one  of  these  pages,  cost  him  seventeen 
years  of  hard  work.  When  the  third  one  was  estab- 
lished, he  said  of  the  book  containing  it,  "  It  may  well 
wait  a  century  for  a  reader,  as  God  has  waited  six 
thousand  years  for  an  observer." 

10.  Galileo.1 — This  famous  Italian  first  used  the  tele- 
scope in  astronomy.      The   first  telescope  was  made 
in  Holland  in  1608 ;  a  vague  report  of  the  invention 
reached  Galileo  the  next  year,  and  from  this  hint,  after 
one  night's  reflection,  he  was  able  to  construct  one 
which  magnified  objects  three  times,  and  he  finally 
made  one  which  magnified  thirty-two  times.     He  dis- 
covered the  moons  of  Jupiter,  the  spots  upon  the  sun, 
and  many  other  wonderful  things.     His  brilliant  dis- 
coveries convinced  the  world  of  the  truth  of  the  Co- 
pernican  theory,  but  brought  on  him  the  condemnation 
of  the  Church  for  teaching  heresies,  and  the  closing 
years  of  his  life  were  saddened  by  its  persecutions. 
Natural  philosophy  is  as  greatly  indebted  to  this  re- 
markable man  as  astronomy. 

11.  Newton? — Within  a  year  of  the  day  on  which 
Galileo  died,  Sir  Isaac  Newton  was  born  in  England. 
While  Newton  did  not  discover  the  law  of  gravitation,  as 
is  sometimes  stated,  yet  he  first  proved  that  the  force 
which  brings  the  apple  to  the  earth  binds  the  planets 
and  sun  into  one  system.3     This  establishment  of  the 


1  Galileo  (Gal-i-lee'o),  1564-1642. 

»  New'ton,  1642-1727. 

8  The  well-known  story  that  while  driven  into  the  country  by  the 
Plague  in  London,  Newton  noticed  an  apple  falling  from  a  tree,  and 
that  this  suggested  the  idea  that  the  motions  of  the  planets  might  be 
controlled  by  the  same  force,  is  worth  remembering.  This  discovery 


INTRODUCTION.  17 


fact  that  gravity  is  the  force  which  controls  the  motions 
of  the  heavenly  bodies  was  of  the  greatest  importance  : 
a  large  part  of  the  science  of  astronomy  depends  upon 
it.1  Newton,  like  Kepler,  was  a  mathematical  astron- 
omer, not  an  observer.  He  discovered  and  proved 
many  other  important  facts  in  astronomy,  besides 
making  many  and  valuable  discoveries  in  natural  phi- 
losophy and  other  sciences.  He  also  occupied  impor- 
tant positions  under  the  English  government.  Sir 
Isaac  Newton  was  probably  the  greatest  scientist  that 
the  world  has  yet  seen.  His  great  work  is  called  the 
"  Principia."  La  Place,2  the  only  man  who  could  have 
disputed  Newton's  pre-eminence  as  a  mathematical  as- 
tronomer, pronounced  this  work  the  greatest  produc- 
tion of  the  human  intellect. 

12.  Modern  Astronomy. — Since  Newton  a  host  of  emi- 
nent astronomers  and  mathematicians  have  given  their 
lives  to  the  advancement  of  our  science.  Every  gen- 
eration and  every  civilized  country  has  furnished  its 
share.  As  it  was  the  earliest  begun,  so  it  is  the  far- 
thest advanced  of  the  sciences.  Its  strides  seem  to  be 
growing  longer  rather  than  shorter.  Our  own  genera- 
tion and  our  living  astronomers  are  inferior  to  none  of 
their  predecessors  in  ability  or  in  the  value  of  their  dis- 
coveries. And  there  is  every  reason  to  expect  these 
discoveries  to  go  on  with  increased  rapidity.  In  the 

was  made  by  Newton  while  he  was  absent  from  London  on  account 
of  the  Plague,  but  the  rest  of  the  story  is  not  supported  by  sufficient, 
evidence;  it  is  not  at  all  improbable,  however. 

1  It  may  not  be  amiss  to  remark  that,  while  the  laws  and  effects  of 
gravity  are  well  known,  the  cause  of  this  force  has  never  been  dis- 
covered. 

2  La  Place  (La-plass'),  a  great  French  mathematician  and  astrono- 
mer, 1749-1827. 

a» 


18  INTRODUCTION. 


astronomical  work  of  the  last  generation  our  own 
country  has  done  its  full  share.  Our  astronomers  and 
observatories  have  no  superiors.  Our  contributions  to 
the  world's  store  of  knowledge  have  been  greater  in 
this  direction  than  in  any  other.  The  history  of  the 
important  discoveries  in  astronomy  made  since  New- 
ton's day  would  fill  a  much  larger  book  than  this.  We 
can  only  give  these  discoveries  in  their  proper  places 
in  a  general  account  of  the  subject. 


General  View  of  the  Heavens, 

13.  Introductory. — If  a  person  will  carefully  watch  the 
heavens,  he  will  see  much  that  will  tend  to  excite  his 
curiosity.     What  are  all  the  glittering  lights?     How 
far  are  they  away  ?    Why  do  they  seem  to  move  around 
him  in  a  circle  every  day?     Why  do  some  of  them 
change  their  position  among  the  others  ?     Many  such 
questions  as  these  will  come  up,  and  the  best  method 
of  arriving  at  a  correct  answer  is  first  to  observe  care- 
fully all  that  can  be  seen.     The  ancients  did  this  much 
more  faithfully  than  we  do,  and  the  various  generations 
of  men  have  accumulated  a  great  number  of  facts  and 
laws  of  which  we  can  now  have  the  benefit.    It  is  the 
object  of  a  book  on  astronomy  to  explain  these  points 
so  that  an  observer  can  better  comprehend  the  causes 
of  what  he  sees.     But  careful  watching  must  accom- 
pany the  study  if  the  phenomena  are  to  be  fully  under 
stood. 

14.  The  Heavens  by  Day. — But  what  can  the  unaided 
eye  see  ?     In  the  daytime  there  is  usually  only  the  sun, 
and  this  presents  the  same  general  appearance  every 


INTRODUCTION.  19 


day.  We  will  find  that  continual  changes  are  taking 
place  on  his  surface,  but  these  changes  are  not  visible 
to  the  eye.  His  position  in  the  heavens  is,  however, 
perceptibly  changing.  Every  one  is  familiar  with  the 
motion  which  occurs  each  day, — his  rising  in  the  east, 
reaching  the  highest  point  at  noon,  and  setting  in  the 
west.  A  careful  observer  will  notice,  besides  this,  a 
change  of  place  at  different  seasons  of  the  year.  He  is 
in  the  south  every  day  at  noon,  but  in  the  summer  he 
is  higher  up  in  the  sky  than  in  the  winter.  It  will  be 
noticed,  too,  that  he  does  not  rise  and  set  in  the  same 
place  through  the  year.  If  the  point  of  setting  be 
noted  every  evening,  beginning  with  the  first  of  the 
year,  it  will  be  found  to  be  moving  towards  the  north 
as  the  winter  progresses.  This  will  go  on  till  the  mid- 
dle of  summer,  when  the  place  of  setting  will  be  far  to 
the  north  of  the  west.  Then  it  will  slowly  change  back 
again  towards  the  south  through  the  fall  and  early 
winter.  So  with  the  time  of  rising  and  setting :  it  will 
be  noticed  that  the  farther  to  the  north  the  sun  rises, 
the  earlier  in  the  day  it  will  rise  and  the  later  it  will 
set.1 

15.  Horizon  and  Zenith. — The  circle  where  the  earth 
and  sky  appear  to  meet  is  called  the  horizon.  On  the 
ocean  it  is  a  perfect  circle,  but  on  land  it  is  broken  up 
with  the  irregularities  of  the  surface.  When  the  sun 
rises  it  passes  above  this  circle,  and  when  it  sets  it  sinks 

1  Let  the  student  carefully  note,  by  reference  to  a  tree  or  some  dis- 
tant object,  the  point  in  the  horizon  where  the  sun  rises  or  sets,  at 
intervals  of  a  week  or  two,  and  this  change  of  place  will  be  readily 
manifest.  He  must  be  careful  to  occupy  the  same  point  of  observa- 
tion at  the  different  times.  Let  him  also  with  a  watch  observe  the 
exact  time,  and  thus  notice  the  gradual  change. 


20  INTRODUCTION, 


below  it.  The  point  in  the  sky  directly  over  the  head 
of  the  observer  is  called  the  zenith.1 

16.  The  Heavens  by  Night. — In  the  night  there  is 
much  more  to  attract  attention  in  the  sky.  The 
moon  seems  to  follow  nearly  in  the  path  of  the  sun. 
If  carefully  observed,  she  will  be  seen  to  change  her 
place  among  the  stars,  being  each  night  a  little  farther 
to  the  east  than  the  preceding.  The  changes  in  her 
appearance  from  crescent-shaped  to  full,  and  from  full 
to  crescent-shaped,  are  also  striking.  There  will  be 
certain  nights  each  month  when  she  cannot  be  seen ; 
after  this  a  glimpse  of  her  can  be  obtained  in  the  west 
just  after  sunset ;  she  will  then  be  crescent-shaped,  and 
her  horns  will  point  directly  away  from  the  sun.  She 
will  then  grow  in  size  for  about  two  weeks,  all  the 
time  appearing  farther  and  farther  away  from  the  sun 
at  sunset,  till  when  quite  full  she  will  rise  in  the  east 
just  as  the  sun  is  setting  in  the  west.  Then  she  will 
go  through  the  changes  in  a  reverse  order  for  two 
weeks  more. 

It  will  also  be  noticed  that  certain  of  the  brighter 
stars  appear,  like  the  moon,  to  change  their  places 
among  the  others.  The  ancients  called  these  planets, 
or  "  wandering  stars."  Those  that  can  readily  be  seen 
by  the  naked  eye  are  Venus,  Mars,  Jupiter,  and  Saturn. 

But  the  great  majority  of  the  stars  preserve  exactly 
their  relative  positions.  They  appear  night  after  night 
looking  precisely  the  same.  A  given  star  will  always 

1  Towards  what  point  will  a  plumb-line,  extended  upwards,  point? 
Does  the  zenith  change  with  a  change  of  position  on  the  earth  ?  Is 
the  sun  ever  seen  in  the  zenith  in  the  northern  hemisphere?  On 
which  side  of  the  zenith  is  the  sun  at  noon  ?  In  what  time  of  year 
does  the  sun  pass  nearest  the  zenith  ? 


INTRODUCTION.  21 


rise  in  the  same  point  in  the  horizon,  though  not  at 
the  same  time ;  it  will  always  follow  in  the  same  path 
throughout  the  year,  and  set  in  the  same  place  in  the 
west. 

But  it  will  be  noticed  that  the  paths  which  different 
stars  describe  are  very  different.  If  we  look  in  a  north- 
erly direction  towards  a  point  nearly  half-way  from  the 
horizon  to  the  zenith,  we  shall  see  a  star  of  medium 
brightness  which  does  not  change  place  at  all ;  it  is  the 
pole-star,  or  Polaris.  Around  this  star  all  the  northern 
heavens  seem  to  revolve  in  circles.  If  these  northern 
or  circumpolar  stars  be  watched,  such  as  are  between 
the  pole-star  and  the  horizon  will  move  towards  the 
east;  such  as  are  on  the  east  of  the  pole  will  ascend; 
such  as  are  above  will  move  westward;  and  such  as  are 
to  the  west  of  the  pole  will  descend.  Those  stars  situ- 
ated a  little  farther  from  the  pole  than  the  northern 
horizon  is  will  just  dip  below  it  and  remain  set  but  a 
short  time.  Those  that  rise  in  the  east  will  be  visible 
just  twelve  hours  and  set  in  the  west ;  they  will  not, 
however,  pass  through  the  zenith,  but  south  of  it, 
always  remaining  the  same  distance  from  the  pole- 
star.  Still  farther  to  the  south  the  stars  will  be  but  a 
short  time  above  the  horizon,  passing  over  from  south- 
east to  southwest.1 

1  In  order  to  obtain  a  correct  idea  of  this  diurnal  motion,  the  stu- 
dent should  watch  stars  in  different  parts  of  the  heavens  at  intervals 
of  a  few  hours,  so  as  to  notice  the  paths  they  are  describing.  It  is 
also  advisable  to  set  a  globe  so  that  the  axis  about  which  it  revolves 
will  point  nearly  to  the  pole-star.  The  horizontal  ring  encircling  the 
globe  will  then  represent  the  horizon.  By  turning  the  globe  on  its 
axis,  it  will  be  seen  that  the  part  around  the  pole  will  not  pass  below 
the  horizon,  and  the  various  circles  of  latitude  will  represent  the 
paths  of  stars  in  different  parts  of  the  sky.  -  Some  portions  around 


22  INTRODUCTION. 

17.  Diurnal  Motion. — This  general  motion  of  the  sun, 
moon,  planets,  and  stars,  which  carries  them  apparently 
around  the  earth  every  twenty-four  hours,  is  called  the 
diurnal  motion.    The  heavens  appear  to  us  to  be  the  con- 
cave surface  of  a  sphere,  called  the  celestial  sphere.    The 
celestial  sphere  and  all  the  heavenly  bodies  revolve 
about  the  earth  every  day,  while  the  sun,  moon,  and 
planets  have  a  separate  motion  of  their  own,  which 
causes  them  to  change  their  places  among  the  stars. 

18.  Cause  of  Diurnal  Motion. — A  quiet  motion  often 
gives   the   impression  of  rest.     A  sailing  vessel  will 
glide  along  through  still  water  so  quietly  that  a  person 
on  board  can  easily  conceive  that  he  is  at  rest  and  sur- 
rounding objects  are  in  motion  in  the  opposite  direc- 
tion.    Now  the  earth  is  turning  on  its  axis  from  west 
to  east  with  a  perfectly  noiseless  and  smooth  motion. 
The  effect  produced   on   us  is  that  all  the   heavenly 
bodies  are  passing  over  from  east  to  ivest.     The  appar- 
ent diurnal  motion  of  the  heavens  is  therefore  due  to 
a  real  motion  of  the  earth.     Instead  of  the  sun,  moon, 
and  stars  rising  above  the  horizon,  the  eastern  horizon 
is  really  falling  away  from  them.     Instead  of  their  set- 
ting, the  western  horizon  is  rising  to  obscure  them. 
The  reason  that  they  appear  to  climb  the  sky  is  because 
the  portion  of  the  earth  on  which  we  are  is  turning 
more  directly  under  them ;  and  the  reason  that  they 
sink  is  because  we  are  revolving   away  from  them. 
All  the  effects  of  diurnal  motion  above  described  are 
readily  explained  by  the  rotation  of  the  earth  on  its 
axis,  this  axis  pointing  nearly  towards  the  pole-star. 


the  south  pole  will  not  pass  above  the  horizon.     There  are  some  verj 
brilliant  southern  stars  that  we  never  see  in  this  latitude. 


INTRODUCTION.  23 

Ir;  some  explanations  it  is  easier  to  consider  that  the 
sun  moves  about  the  earth,  as  it  seems  to  do.  When 
we  speak  in  this  way,  it  must  be  remembered  that  we 
refer  to  the  apparent  and  not  the  real  motion.1 

19.  Celestial  Measures. — The  heavenly  bodies  being 
apparently  on  the  inner  surface  of  a  sphere,  the  line 
joining  their  positions  on  this  sphere  is  an  arc  of  a 
circle.  Hence  we  do  not  measure  distances  in  the 
heavens  by  miles  or  other  linear  units,  but  by  circular 
measure.  Every  circle  is  divided  into  three  hundred 
and  sixty  degrees  (°),  each  degree  into  sixty  minutes  ('), 
and  each  minute  into  sixty  seconds  (").  The  distance 
from  the  zenith  to  the  horizon  is  a  quarter  of  a  circle, 
or  ninety  degrees.  It  is  well 
for  the  student  to  have  a  correct 
idea  of  the  size  of  small  meas- 
ures in  the  heavens.  The  fol- 
lowing will  aid  in  obtaining  it. 
There  are  two  stars  which  con- 
tinually point  to  the  pole-star. 
They  are  two  of  the  seven  Fl0'1' 

which  form  what  is  often  called  the  Dipper,  revolving 
continually  about  the  pole-star  and  just  touching  the 
northern  horizon.  These  two  "pointers"  are  just 


1  In  the  summer  of  1881  there  was  a  bright  comet,  the  tail  of  which 
pointed  nearly  to  Polaris.  It  partook  of  the  diurnal  motion  of  the 
heavens,  and  being  near  the  pole-star  was  seen  all  night.  When 
in  the  northwest  in  the  evening,  its  tail  pointed  upwards  and  to  the 
right.  When  it  got  around  to  the  northeast  in  the  morning,  the  tail 
•pointed  upwards  and  to  the  left.  Many  people  who  saw  it  in  both 
of  these  positions,  not  understanding  about  the  diurnal  motion, 
thought  there  were  two  comets.  Let  the  student  think  of  this  matter 
till  he  sees  how  it  was  that  the  comet  thus  changed  the  direction  of 
its  tail  with  reference  to  the  horizon. 


24  INTRODUCTION. 


about  five  degrees  apart.  The  diameter  of  the  sun 
and  that  of  the  full  moon  are  each  about  half  a  degree 
or  thirty  minutes  long.  If  two  stars  are  nearer  to- 
gether than  three  or  four  minutes,  they  will  appear  as 
one  to  the  eye.1 

20.  The  Heavens  at  the  Equator  and  at  the  Poles. — As 
the  observer  changes  his  position  on  the  earth,  the  ap- 
pearance of  the  heavens  will  also  change.  If  he  move 
eastward  or  westward,  his  horizon  will  move  the  same 
way,  and  the  time  of  rising  and  setting  of  the  stars 
will  vary.  If  the  movement  be  eastward,  the  same 
stars  will  pursue  the  same  course  through  the  sky,  but 
they  will  rise  earlier  and  set  earlier ;  if  westward,  the 
reverse  will  be  the  case.  If,  however,  the  observer 
move  towards  the  north  or  south,  the  whole  aspect  of 
the  heavens  will  change. 

The  reason  that  the  pole-star  does  not  seem  to  move 
is  because  the  axis  about  which  the  earth  revolves 
points  almost  directly  towards  it.2  There  is  no  change 
of  the  horizon  with  reference  to  it.  To  an  observer  at 
the  equator  the  pole-star  would  be  at  the  horizon,  be- 


1  The  following  additional  measurements  will  assist  in  estimating 
distances.  The  stars  may  be  found  on  a  map  or  globe,  or  some  one 
knowing  them  may  point  them  out  in  the  heavens.  The  extreme 
stars  of  the  three  in  the  belt  of  Orion  are  about  three  degrees  apart ; 
Castor  and  Pollux  about  four  degrees.  Near  Vega  is  a  faint  star, 
which  by  a  good  eye  can  be  seen  to  be  made  up  of  two  stars.  They 
are  three  and  a  half  minutes  apart.  The  height  of  the  pole-star  above 
the  horizon  is  about  equal  to  the  latitude  of  the  place.  In  the  Middle 
States  this  is  nearly  forty  degrees. 

8  The  axis  of  the  earth  does  not  point  directly  towards  the  pole-star, 
Vat  about  one  and  a  half  degrees  from  it.  The  pole-star,  therefore, 
describes  a  small  circle  about  the  pole  of  the  heavens,  though,  roughly 
•peaking,  it  may  be  said  to  correspond  with  it. 


INTRODUCTION.  25 


cause  the  axis  of  the  earth  is  pointing  in  that  direction. 
If  a  globe  be  set  with  its  axis  horizontal,  it  will  show 
the  motion  of  the  heavenly  bodies  to  a  person  at  the 
equator.  The  stars  that  rise  in  the  east  will  pass  di- 
rectly overhead  and  set  in  the  west ;  every  star  will  be 
just  twelve  hours  above  the  horizon ;  those  around  the 
poles  will  describe  small  circles,  those  farther  away 
larger ;  there  will  be  no  stars  that  never  rise,  and  none 
that  never  set. 

As  the  person  moves  towards  the  north  pole,  the  pole- 
star  will  rise  above  the  horizon,  its  height  being  equal 
to  the  latitude  of  the  person ; l  that  is,  if  the  observer 
is  at  latitude  40°,  as  at  Philadelphia,  the  pole-star  will 
be  forty  degrees  above  the  horizon.  When  the  ob- 
server reaches  the  pole,  the  pole-star  will  be  in  his 
zenith ;  the  heavens  seem  to  move  as  in  the  case  of  a 
globe  with  its  axis  vertical;  only  one-half  the  stars 
will  be  ever  visible;  those  in  the  horizon  will  con- 

1  The  elevation  of  the  pole-star  may  be  shown  to  be  equal  to  the 
latitude  by  the  aid  of  Fig.  2. 

Let  BAG  .be  a  meridian  of  the  earth,  P  the  north  pole,  and  E  the 
point  where  the  meridian  cuts 
the  equator.  The  axis  of  the 
earth,  OP,  will  cut  the  celestial 
sphere  at  a  point,  P',  very  near 
the  pole-star.  Let  A  be  the  posi- 
tion of  the  observer  on  the  earth. 
Then  the  arc  EA,  or  the  angle 
EGA,  is  the  latitude  of  the  place  J  °  ° 

of  the  observer,  and  BOP  is  the 

elevation  of  the  pole  of  the  heavens  above  the  horizon.  We  wish  to 
prove  that  EOA  =  BOP. 

BOA  is  a  right  angle,  as  is  also  POE,  because  the  equator  is  ninety 
degrees  from  the  pole.  Hence  BOA  =  POE.  Taking  from  these 
equals  the  angle  POA,  we  have  BOP==AOE,  which  is  what  we 
wished  to  prove. 


26  INTRODUCTION. 

tinually  skirt  around  the  horizon ;  none  will  ever  rise, 
and  none  set. 

The  same  changes  would  be  noticed  if  the  observer 
moved  southward  from  the  equator,  except  that  there 
is  no  star  to  mark  the  position  of  the  south  pole. 


Usefulness  of  Astronomy. 

21.  Astronomy,  besides  being  a  very  grand  and 
interesting  science,  has  great  practical  usefulness. 

Every  day  there  is  telegraphed  over  the  country  from 
the  Washington  and  other  observatories  the  accurate 
time  of  noon ;  this  is  determined  by  astronomical  ob- 
servations, without  which  it  would  be  almost  impos- 
sible to  keep  our  clocks  and  watches  correct. 

Every  captain  of  a  vessel  when  he  starts  out  on  a 
long  voyage  takes  with  him  a  chronometer1  which  has 
been  previously  tested  at  an  observatory,  and  a  nautical 
almanac?  in  which  the  positions  of  the  sun,  moon,  and 
principal  stars  are  given  with  great  accuracy.  With 
these  and  some  simple  observations  he  is  a*ble  to  tell 
his  position  on  the  ocean  and  thus  to  direct  his  move- 
ments. 

The  basis  of  our  calendar  is  astronomical.  The 
lengths  of  the  year,  month,  and  day  are  governed  by 
phenomena  of  the  heavenly  bodies,  and  are  determined 
by  observations  of  them.  Our  common  almanacs  are 
calculated  from  the  nautical  almanacs,  which  are  issued 
from  the  national  observatories. 

1  A  clock  swinging  in  rings,  so  that  the  motion  of  the  vesse"  will 
not  affect  it. 

2  This  will  be  further  explained  on  page  169. 


INTRODUCTION.  27 


All  the  maps  of  the  surface  of  the  earth  are  depend- 
ent for  their  accuracy  on  astronomical  observations; 
the  methods  of  finding  latitude  and  longitude  are 
largely  astronomical. 

Astronomy  is  also  a  help  to  geology,  to  meteorology, 
and  to  other  sciences. 

Hence  we  see  that  it  is  one  of  the  very  practical 
sciences;  and  it  will  probably  be  found  that  some  of 
its  researches,  which  do  not  now  seem  to  be  of  any 
use  to  man,  will  in  the  future  be  in  some  way  closely 
related  to  his  welfare. 

It  will  be  of  use  to  students  also,  if  they  study  it 
rightly,  to  teach  habits  of  observation,  to  strengthen 
their  powers  of  thought,  and  to  give  correct  ideas  of  the 
method  by  which  the  Creator  of  the  universe  works. 


PART  L 

THE  SOLAR  SYSTEM. 


CHAPTER  L 

VIEW   OF   THE    SOLAR   SYSTEM. 


22.  Parts  of  the  Solar  System.—  The  group  of  bodies 
to  which  the  earth  belongs  is  called  the  solar  system. 
It  consists  of  the  sun,  the  planets,  their  satellites  or 
moons,  the  comets,  and  meteoroids.1      The  earth  is 
one  of  the  planets,  and  the  moon  one  of  the  satellites. 
These  bodies  are  closely  connected  with  one  another, 
and,  comparatively  speaking,  are  close  together.     The 
sun   is  very  much  the   largest   and   most  important 
member  of  the  system  :  hence  the  name  solar2  system. 
The  stars  are  all  situated  at  immense  distances  from 
us,  and,  aside  from  their  light,  exert  little  or  no  in- 
fluence upon  us. 

23.  Arrangement  of  the  Solar  System.  —  The  sun  is  the 
centre  of  the  solar  system,  and  about  it  all  of  the  other 

1  "  Shooting  stars"  are  meteoroids  which  have  come  into  our  at- 
mosphere. 

a  Solar,  from  Latin  sol,  the  sun. 

3*  29 


30 


ASTRONOMY. 


members  revolve.  The  time  that  it  takes  one  of  these 
bodies  to  revolve  about  the  sun  is  called  its  year.  If 
an  observer  could  be  at  the  sun  and  watch  the  other 
members  of  the  solar  system,  they  would  revolve  about 
him  in  apparent  circles,  just  as  we  see  the  moon  re- 
volving about  the  earth.  But  from  one  of  the  planets 
these  motions  do  not  seem  so  simple,  and  it  was  a  long 
time  before  men  found  out  that  the  earth  and  the  rest 
of  these  bodies  revolve  about  the  sun. 


FIG.  3.— THE  ORBITS  OF  MARS,  THE  EARTH,  AND  VENCS.  One  inch  =  100,0(10,000  miles. 
The  arrows  show  the  direction  in  which  the  planets  move,  as  seen  from  the  north  side 
of  their  orbits. 

The  path  in  which  a  body  moves  about  the  sun  is 
called  its  orbit.  Fig.  3  shows  the  orbits  of  the  earth 
and  the  planets  next  to  it  on  either  side,  Mars  and 


GENERAL    VIEW  OF  THE  SOLAR   SYSTEM.       31 

Venus.  Those  planets  whose  orbits  are  inside  of  the 
earth's  orbit,  as  Venus,  are  called  inferior  planets,  be- 
cause they  are  nearer  to  the  sun.  Those  outside  are 
called  superior  planets. 

24.  Positions  and  Apparent  Motions. — When  a  heav- 
enly body  is  on  the  side  of  the  earth  opposite  to  the  sun, 
it  is  said  to  be  in  opposition  ;  thus,  if  Mars  is  at  M,  with 
the  earth  at  E,  Mars  is  in  opposition.  When  a  heav- 
enly body  and  the  sun  are  on  the  same  side  of  the 
earth,  the  body  is  in  conjunction  ;  thus,  if  Mars  is  at  M',1 
with  the  earth  at  E,  Mars  is  in  conjunction.  It  is  evi- 
dent from  the  figure  that  an  inferior  planet  has  two 
conjunctions.  With  the  earth  at  E,  Venus  at  V  is  in 
inferior  conjunction,  but  at  V  is  in  superior  conjunction. 
If  in  going  between  the  earth  and  the  sun  Venus 
should  happen  to  pass  directly  across  the  face  of  the 
sun,  it  would  be  called  a  transit.2  This  rarely  happens ; 
the  inferior  planets  usually  cross  a  little  below  or  above 
the  sun.  A  superior  planet  may  be  seen  at  any  height 
in  the  heavens;  it  may  be  in  opposition  to  the  sun, 
when  it  would  rise  about  the  time  the  sun  sets,  and 
would  shine  all  night.  An  inferior  planet  can  never 
be  in  opposition  to  the  sun,  but  in  revolving  about  the 
sun  seems  to  us  to  pass  back  and  forth,  from  one  side 
of  the  sun  to  the  other,  as  Fig.  3  shows  is  the  case  with 
Venus.  An  inferior  planet,  then,  is  never  far  from  the 
sun  ;  it  is  only  seen  a  little  while  after  sunset  or  before 
sunrise.  When  Venus  is  at  V"  or  V",  with  the  earth 
at  E,  it  seems  to  be  farthest  from  the  sun ;  it  is  then 
said  to  be  at  its  greatest  elongation.  The  planets  all 

1  M'  is  read  M  prime;  V,  V  prime;  V",  V  second;   V",  V 

third,  etc. 

2  Can  a  superior  planet  ever  transit  ? 


32  ASTRONOMY. 


move  about  the  sun  in  the  same  direction,  from  west 
to  east.  To  an  observer  north  of  them  (as  anywhere 
in  the  United  States  north  of  Florida)  they  would  seem 
to  move  roin  the  right  over  to  the  left,  or  in  a  direc- 
tion opposite  to  the  motion  of  the  hands  of  a  clock.1 
Although  the  planets  always  move  around  the  sun  in 
the  same  direction,  our  position  upon  the  earth  makes 
them  seem  to  move  differently  sometimes.  With  the 
earth  at  E,  Yenus  seems,  while  moving  from  V"  to 
Y/r/,  to  move  in  the  proper  direction,  from  right  to 
left,  but  while  moving  from  V"  to  V",  across  between 
us  ?,nd  the  sun,  it  seems  to  move  in  the  opposite  direc- 
tion.2 This  is  called  its  retrograde  (backward)  motion. 
A  superior  planet  retrogrades  when  the  earth  passes 
between  it  and  the  sun.  The  earth  leaves  the  planet 
behind,  and  it  seems  to  move  backward,  just  as  trees 
seem  to  move  backward  when  we  pass  them  in  the 
cars.  If  we  imagine  ourselves  at  E,  watching  Yenus 
pass  us,  or  Mars  as  we  pass  him,  it  will  be  clear. 

25.  Shapes  of  the  Orbits. — The  orbits  of  the  planets 
are  not  circles,  but  ellipses.  An  ellipse  is  an  oblong 
curve,  so  made  that  the  sum  of  the  distances  from  any 

1  This  motion  must  not  be  confounded  with  the  apparent  diurnal 
motion  of  the  heavenly  bodies.     All  of  the  planets  and  stars  seem  to 
move  every  night  from,  east  to  west,  which,  as  has  been  explained,  is 
caused  by  the  revolution  of  the  earth  upon  its  axis.     But  the  motion 
here  referred  to  is  one  which  the  planets  have  in  the  opposite  direction 
among  the  stars,  just  as  the  moon  moves  to  the  east  among  the  stars, 
although  it  rises  and  sets  with  them.     The  planets  move  more  slowly 
than  the  moon,  but  if  one  of  them  be  watched  from  night  to  night, 
its  motion  eastward  among  the  stars  may  be  seen.      It  is  very  im- 
portant to  have  this  matter  perfectly  clear. 

2  For  the  sake  of  simplicity  the  earth  is  here  supposed  to  be  station- 
ary ;  the  earth's  motion  really  shortens  very  much  the  time  of  retrogra- 
dation. 


GENERAL    VIEW  OF  THE   SOLAR   SYSTEM.      33 


point  of  it  to  two  fixed  points  is  always  the  same.  Fig.  4 
represents  an  ellipse.  The  sum  of  ES  and  EF  is  just 
equal  to  the  sum  of  E'S  and  E'F.1  If  the  ends  of  a 
string  be  fastened  at  two  points  (S  and  F)  upon  a  table, 
so  as  to  lie  loosely  between  them,  and  a  pencil  held 
against  the  string  so  as  to  stretch  it  (as  at  E)  be  moved 


FIG.  4. — ELLIPSE. 


FIG.  5. — PARABOLA. 


along,  it  will  mark  an  ellipse.  S  and  F  are  called  the 
foci  (fo'si).  In  the  orbits  of  the  planets  the  sun  is  al- 
ways at  one  focus  (fo'kus).  If  the  foci  are  nearer  to 
the  centre  C,  the  ellipse  is  nearer  circular.  The  eccen- 
tricity of  an  ellipse  is  the  distance  OS  divided  by  CP ; 
it  is  usually  expressed  in  a  decimal  fraction :  the  eccen- 
tricity in  Fig.  4  is  .8,  or  £.2  The  eccentricity  of  an 
ellipse  shows  whether  it  is  nearly  circular  or  more  ob- 
long. The  orbits  of  the  planets  have  very  little  eccen- 
tricity, as  the  table  in  Art.  27  shows.  It  must  be  re- 
membered, then,  that  the  elliptical  shape  of  a  planetary 


1  Measure  the  lines  in  the  figure,  and  see  if  SB  and  EF  taken  to- 
gether are  equal  to  SB'  and  E'F.  Try  the  sum  of  the  distances  to 
any  other  point  on  the  curve. 

»  Measure  CS  and  CP,  and  see  if  OS  is  .8  (or  f )  of  CP. 


34  ASTRONOMY. 

orbit,  as  shown  in  Fig.  4,  is  greatly  exaggerated.  An 
exact  figure  of  a  planet's  orbit  could  not  be  distin- 
guished by  the  eye  from  a  circle.  Fig.  3  shows  the 
real  shapes  of  the  orbits  of  Mars,  Earth,  and  Venus. 
If  the  sun  and  the  other  orbits  be  covered,  no  one  of 
these  can  be  distinguished  from  a  circle.  That  point 
of  a  planet's  orbit  which  is  nearest  to  the  sun  is  its 
perihelion;1  the  point  which  is  farthest  from  the  sun  is 
its  aphelion.2  In  Fig.  4,  P  is  the  perihelion,  and  A  the 
aphelion.  The  difference  between  the  distances  of 
these  two  points  from  the  sun  may  be  very  consider- 
able, even  if  the  orbit  does  seem  to  be  almost  a  circle. 
In  the  case  of  the  earth  the  difference  is  three  millions 
of  miles,  and  with  most  of  the  other  planets  the  differ- 
ence is  greater.  Some  of  the  comets  are  thought  to 
move  not  in  ellipses,  but  in  parabolas?  The  two  sides 
of  this  curve  (Fig.  5)  keep  separating  farther  and  far- 
ther forever.  The  parabola  is  not  a  closed  curve  like 
the  circle  and  the  ellipse. 

26.  Characteristics  of  all  the  Planets. — Next  to  the  sun 
the  planets  are  the  most  important  parts  of  the  solar 
system.  They  are  alike  in  many  points.  Besides 
moving  about  the  sun  in  the  same  direction  in  ellip- 
tical orbits,  they  all  seem  to  revolve  upon  their  axes  in 
the  same  direction,  giving  them  all  day  and  night. 
Their  paths  all  lie  nearly  in  the  same  plane.  They 
are  all  of  the  same  shape.  They  all  shine  by  reflected 
sunlight. 

1  Perihelion,  from  the  Greek  peri,  near,  and  helios,  the  sun. 

2  Aphelion,  from  the  Greek  apo,  from,  and  helios,  the  sun. 

8  The  paraVola  is  so  drawn  that  every  point  of  the  curve  is  equally 
distant  from  a  fixed  point  and  a  fixed  straight  line.  As  in  Fig.  5, 
CD  and  OS  are  equal ;  S  is  the/ocws,  and  DD'  the  directrix. 


GENERAL    VIEW  OF  THE  SOLAR   SYSTEM.      35 


27.  Statistics  of  the  Sun  and  Planets. 


Name. 

Average 
dist.  from 
the  sun. 

Diameter  in  miles. 

Length  of  year. 

Length  of  day. 

Mass  (times  the  weight 
of  the  earth). 

Density  (times  the 
weight  of  water). 

Eccentricity  of  orbit. 

Millions  of 
miles. 

I  Times  the 
|  earth's  dist. 

Sun 

866,000 

3000 
7630 
7918 
4200 

20 
to 
300  (?) 
86,000 
73,000 
32,000 
35,000 

days. 
88 
225 
365J 
687 
years. 
3 
to 
7 
12 
29* 
84 
164* 

25  days. 

88  days  ? 
•^25  days? 
23h.  56m. 
24h.  37m. 

unknown. 

9h.  55m. 
lOh.  14m. 
unknown, 
unknown. 

330,000 

f 

unknown. 

312 
93 

}? 

11 

6| 
4f 

1 

unknown. 
If 

ij 
1} 

Mercury  

36 
67 
93 
142 

200 
to 
325 

483 
886 
1,782 
2,790 

* 

IJ 

2 
to 

? 

,99J 

30 

0.2056 
0.0068 
0.0168 
0.0933 

0.02 
to 
0.38 
0.0483 
0.0560 
0.0464 
0.0090 

Venus  
h  arth 

Mars  

Planetoids.  ..J 
Jupiter  

Saturn 

Uranus  

Neptune  

This  table  is  not  to  be  committed,  as  the  most  im- 
portant of  these  statistics  will  be  given  in  round  num- 
bers in  connection  with  the  separate  planets,  but  some 
of  its  striking  facts  should  be  noticed.  The  sun  is  by 
far  the  largest  body  in  the  solar  system.  His  mass  is 
seven  hundred  times  that  of  all  of  the  planets  together. 
The  planets  are  divided  into  three  groups.  Nearest 
the  sun  are  four  small  planets,  not  differing  very  greatly 
in  size ;  of  these  the  earth  is  the  largest.  Next  to  these 
is  a  large  number  of  ve^  small  planets,  or  planetoids. 
Then  come  four  giant  planets,  which  in  several  re- 
spects resemble  one  another.  The  four  small  planets 
are  of  heavy  material;  the  sun  and  the  four  large 
planets  are  all  about  as  light  as  water.  Two  of  the 
four  small  planets  have  days  about  twenty-four  hours 
long,  while  all  of  the  large  planets  whose  axial  rev- 
olutions have  been  determined  have  days  of  only 


ASTRONOMY. 


ten  hours.     Figs.  6  and  7  will  assist  in  giving  clear 
ideas  of  the  sizes  and  distances  of  the  planets.      In 

Fig.  7  two  of  the 
planetoids  are  put 
between  Mars  and 
.Jupiter.  These 
planetoids  are  very 
small  planets.  They 
all  come  between 
Mars  and  Jupiter, 
and  are  close  to- 
gether. Little  is 
known  about  them. 
The  planetoids  and 
the  two  farthest 
planets,  Uranus 
and  Neptune,  were 
unknown  to  the 
ancients. 

28.  Satellites.  — 
All  of  the  principal 
planets  except  the 
two  inner  ones, 
Venus  and  Mer- 
cury, have  one  or  more  satellites.  The  earth  has  one 
satellite,  the  moon;  and  the  satellites  of  the  other 
planets  are  often  called  their  moons.  These  satellites 
all  revolve  around  their  planets,  just  as  the  planets  re- 
volve about  the  sun,  and  are  carried  with  them  by  the 
planets  in  their  journey  about  the  sun. 

29.  Kepler7 s  Laws. — As  has  been  said,  Kepler  discov- 
ered three  important  laws,  by  which  the  motion  of  all  the 
planets  and  their  satellites  is  interpreted.  These  are : 


FIG.  6.— THE  COMPARATIVE  SIZE  OF  THE  PLANETS. 


GENERAL    VIEW   OF  THE  SOLAR   SYSTEM.      37 

I.   The  planets  move  in  ellipses,  with  the  sun  in  one  focus. 

Before  the  discovery  of  this  law,  astronomers  had 
always  assumed 
that  the  planets 
move  in  circles, 
and  it  must  not  be 
forgotten  that  these 
ellipses  are  almost 
circles.  When  it  is 
at  perihelion,  or 
nearest  the  sun,  a 
planet  moves  fast- 
est; if  it  did  not, 
the  increased  at- 
traction of  the  sun 
would  cause  the 
planet  to  fall  into 
it.  This  is  be- 
tween P2  and  P3  in 
Fig.  8 ;  but  as  the 
planet  moves  from 
P3  to  P4,  the  sun's 
attraction,  pulling 

it    back,    makes   its    Fia-  7.— THE  COMPARATIVE  SIZES  OF  THE  SUN,  AS  SEBN 
, .  -,  ,  FROM  THE  DIFFERENT  PLANETS. 

motion  slower  and 

slower,  until  between  P4  and  P5  it  is  slowest  of  all.  If 
this  were  not  the  case,  the  sun's  attraction  upon  it  here 
would  be  too  weak  to  hold  it  in  its  place,  and  it  would 
fly  off  into  space.  As  it  turns  and  passes  through  P 
and  P1,  the  sun's  attraction  pulls  it  forward  and  contin- 
ually increases  its  velocity,  so  that  at  perihelion  the 
planet's  motion  is  swift  enough  to  carry  it  past  the  sun 
without  falling  into  it. 

4 


38  ASTRONOMY. 


II.  TJie  radius-vector  of  each  planet  sweeps  over  equal  areas 
in  equal  times. 

The  radius-vector  is  the  line  drawn  from  the  sun  to 
any  point  of  the  orbit,  as  SP,  SP1,  SP2,  etc.,  in  Fig.  8, 


P  P' 

Fia.  8. — TLLUSTPATIXO  KEPLER'S  PKCOXD  LAW. 

In  this  figure  suppose  that  PP1,  P2P3,  and  P4P5  each 
represent  the  path  of  a  planet  for  two  weeks.  Then 
the  three  shaded  parts  will  be  equal  in  area. 

III.  The  squares  of  the  times  of  revolution  of  two  planets 
are  proportional  to  the  cubes  of  their  distances  from  the  sun. 

To  illustrate  this  law,  let  us  compare  the  times  and 
distances  of  Mercury  and  Mars,  from  the  table  on 
page  35,  and  by  this  law  we  shall  have : 

882       :    6872  :  :  36,000,0003  :  141,000,0003 

/Mercur.v'sX  /Mars's  \  /Mercnry's\  /    Mars's  \ 

V     period.    /          \  period.;  V  distance.  /  V  listancej 

If  this  be  worked  out,  the  product  of  the  means  will 
be  found  to  be  nearly  equal  to  the  product  of  the  ex- 


GENERAL    VIEW  OF  THE  SOLAR   SYSTEM.      39 

tremes.1  The  same  proportion  will  be  true,  for  any 
other  pair  of  planets.  Observation  has  fixed  the  times 
of  revolution  of  the  planets  very  exactly,  and  when 
the  distance  of  the  earth  from  the  sun  is  found,2  the 
third  law  enables  us  to  find  the  distance  of  any  other 
planet  from  the  sun  by  the  proportion  : 

Square  of        .          square  of         .   .  cube  of  .  cube  of 

earth's  period     •     planet's  period     •  •     earth's  distance     •     planet's  distance. 

Knowing  the  first  three  terms  of  this  proportion,  the 
last  is  found  by  arithmetic.  This  is  the  method  used 
by  astronomers  to  find  the  distances  of  the  planets. 

It  also  follows  from  this  law  that  the  planets  near 
the  sun  move  much  faster  than  the  distant  ones. 
The  table  shows  that  Neptune  is  eighty  times  as  far 
from  the  sun  as  Mercury,  and  its  orbit  is  then  eighty 
times  as  long.  But  it  takes  Neptune  seven  hundred 
times  as  long  to  complete  its  circuit.  Mercury  must 
move  nearly  nine  times  as  fast  as  Neptune.  Every 
planet  moves  faster  than  the  planets  outside  of  it.  If 
it  did  not,  it  could  not  keep  from  being  pulled  in  to- 
wards the  sun  by  his  greater  attraction. 

30.  Ecliptic. — As  has  been  said,  the  earth  revolves 
about  the  sun  once  a  year,  but,  as  in  all  such  cases,  it 
seems  to  us  that  the  earth  is  stationary,  and  that  the 
sun  moves  about  it.  The  apparent  yearly  path  of  the 


1  The  products  will  not  be  found  to  agree  exactly,  chiefly  because 
the  distances  and  times  used  above  are  not  quite  exact.  If  the  exact 
distances  and  times  were  used,  the  agreement  would  be  still  a  little 
imperfect,  because  the  different  planets  influence  the  motions  of  one 
another  slightly. 

8  The  method  of  finding  the  distance  from  the  earth  to  the  sun  will 
be  explained  in  chapter  iii. 


40  ASTRONOMY. 

sun  among  the  stars  is  called  the  ecliptic.1  The  earth's 
axis  is  not  perpendicular  to  the  plane  in  which  the  sun 
moves,  but  is  inclined  to  it.  The  angle  between  the 
ecliptic  and  the  equator  is  23J  degrees.  Fig.  9  shows 
this  leaning  of  the  earth's  axis.  SS'  is  the  ecliptic. 


» 


Fio.  9.— THE  ECLIPTIC. 

EQ  is  the  equator.  The  plane  of  the  ecliptic  cuts  the 
earth  along  TT'.  The  angle  EVT  is  the  angle  of 
23|  degrees.  When  the  sun  is  at  S  it  is  directly  over 
T,  which  is  23J  degrees  south  of  the  equator.  This  is 
the  winter  solstice ; 2  it  comes  on  the  21st  of  December. 
This  is  the  shortest  day  of  the  year,  the  sun  being  far- 
thest south.  As  the  sun  moves  around  from  S  towards 
S'  it  shines  directly  down  upon  the  line  TVT',  and  is 
getting  farther  north,  nearer  the  equator.  On  the  20th 
of  March  the  sun  is  half-way  from  S  to  S',  and  then 
shines  directly  down  upon  the  equator  at  V.  This  is  the 
vernal  equinox  f  or  spring  equinox.  On  the  21st  of  June 

1  So  named  because  eclipses  can  occur  only  when  the  moon  is  near 
this  line. 

2  Sol'stice,  from  the  Latin  words  sol,  the  sun,  and  sfo,  to  stand, 
because  the  sun  seems  to  stand  still  here  a  short  time  before  turning 
to  the  north. 

*  E'qui-nox,  from  the  Latin  words  equus,  equal,  and  nox,  night, 
because  the  nights  and  days  are  here  equal.  Vernal,  from  the  Latin 
adjective  vernalis,  spring. 

The  dates  of  the  solstices  and  equinoxes  may  vary  a  day,  because 
365  or  366  days  do  not  make  an  exact  year. 


GENERAL    VIEW  OF  THE  SOLAR   SYSTEM.      41 


the  sun  is  at  S',  the  summer  solstice,  and  is  now  farthest 
north,  being  directly  over  T'.  Half- way  from  S'  to  S, 
on  September  22,  the  sun  again  crosses  the  equator, 
giving  us  the  autumnal  equinox. 

The  student  must  again  carefully  distinguish  this 
motion  of  the  sun  among  the  stars  from  its  apparent 
daily  motion  from  east  to  west.  If  the  stars  could  be 
seen  in  daytime,  the  sun  would  be  seen  to  be  slowly 
moving  among  them  towards  the  east,  just  as  the  moon 
does  at  night ;  it  is  this  path  that  is  the  ecliptic. 

The  ecliptic  is  divided  into  twelve  equal  arcs  of  30 
degrees  each,  called  signs.  They  begin  at  the  vernal 
equinox,  and  take  their  names  from  the  names  of  twelve 
constellations,  or  groups  of  stars.  Their  names — which 
are  all  Latin — and. symbols  are  these : 

From  the 
Vernal  Equinox. 

A'RI-KS,  <y>  (the  ram)        .      , .        •  '  .  •  0°  to    30° 

TAU'RUS,  &  (the  bull)       .        .        .  .  30°  to    60° 

GEM'I-NI,  n  (the  twins)    .        .        .  .  60°  to    90° 

CAN'CER,  05  (the  crab)       .        .        .  '.  90°  to  120° 

LE'O,  St  (the  lion)     .        .        .        .  .  120°  to  150° 

VIR'GO,  v%  (the  virgin)      .         .         .  .  150°  to  180° 

LI'BRA,  =&  (the  balance)    ....  180°  to  210° 

SCOR'PIO,  n\,  (the  scorpion)        .        .  .  210°  to  240° 

SAGITTARIUS,  /  (the  archer)  .        .  .  240°  to  270° 

CAPRICOR'NUS,  VJ  (the  goat)     .        .  .  270°  to  300° 

AQUA/RIUS,  c#  (the  waterman)  \  .  300°  to  330° 

PISCES,  X  (the fishes)      .        .  _.  .  330°  to  360° 

The  sun  enters  the  sign  Aries  at  the  time  of  the  ver- 
nal equinox,  about  March  20,  and  about  a  month  later 
enters  the  second  sign,  Taurus,  and  so  on  through 
them  all  during  the  year.  These  signs  and  their  sym- 
bols are  in  the  first  part  of  our  common  almanacs. 

81.   The  Celestial  Equator. — The  celestial  equator  is  a 


42  ASTRONOMY. 


great  circle  around  the  heavens,  right  above  the  equa- 
tor on  the  earth.  It  cuts  the  ecliptic  at  the  equinoxes, 
making  an  angle  with  it,  of  course,  of  23  J  degrees.  If 
the  equator  were  visible  in  the  sky,  it  would  appear  as 
an  arch,  passing  across  our  southern  sky,  cutting  the 
horizon  just  east  and  west  of  us.  The  path  of  the  sun 
on  March  20,  or  September  22,  is  on  the  equator.  In 
summer  the  sun's  path  is  higher  up  in  the  sky  than  the 
equator ;  in  winter  it  is  lower. 

Latitude  and  longitude  are  used  to  fix  the  position 
of  places  on  the  earth,  and  in  the  same  way  places  in 
the  sky  are  located;  but,  unfortunately,  astronomers 
use  other  names  than  latitude  and  longitude  to  indicate 
corresponding  distances.  The  distance  of  a  star  north 
or  south  of  the  equator  is  called  its  declination.  In- 
stead of  the  meridian  of  Greenwich  or  Washington  to 
reckon  longitude  from,  the  meridian  passing  through 
the  vernal  equinox  is  used.  And  the  distance  that  a 
star  is  east  of  the  vernal  equinox  is  its  right  ascension.1 
Both  declination  and  right  ascension,  like  latitude  and 
longitude,  are  reckoned  by  degrees. 

1  Declination,  like  latitude,  is  measured  both  north  and  south  from 
the  equator  to  the  poles,  but  right  ascension  is  measured  around  by 
the  east  only.  So  that  a  heavenly  body  may  have  any  right  ascen- 
sion up  to  360°. 

What  is  the  greatest  possible  declination  that  any  point  can  have  ? 
where  is  that  point  ?  What  is  the  dec.  of  the  sun  on  June  21  ?  on 
September  22?  What  is  the  dec.  of  your  zenith  ?  (See  Fig.  2). 

What  is  the  K.  A.  (right  ascension)  of  the  sun  on  March  20?  on 
June  21  ?  on  December  21  ?  In  which  sign  is  the  sun  when  his  R.  A. 
is  50°  ?  when  it  is  140°  ?  250°  ?  When  the  sun's  K.  A.  is  110°  is  its  dec. 
north  or  south  ?  when  its  K.  A.  is  180°  ?  when  it  is  300°  ?  What  is  the 
sun's  dec.  when  its  K.  A.  is  90°?  when  180°?  when  270°?  when  360°? 

Bight  ascension  is  usually  reckoned  in  hours  by  astronomers,  1 
hour  being  15  degrees. 


GENERAL    VIEW  OF  THE  SOLAR   SYSTEM.      43 

32.  The  Zodiac.1 — The  zone  of  the  heavens,  extending 
about  eight  degrees  on  each  side  of  the  ecliptic,  is  called 
the  zo'diac.  It  too  is  divided  into  twelve  signs,  which 
have  the  same  names  and  order  as  the  signs  of  the 
ecliptic.  These  signs  roughly  coincide  with  twelve 
constellations,  or  groups  of  stars,  and  it  was  to  these 
constellations  that  the  ancients  gave  the  names  Aries, 
Taurus,  etc.  When  these  names  were  given,  the  sun 
entered  the  constellation  Aries  at  the  time  of  the  ver- 
nal equinox,  and  the  signs  of  the  ecliptic,  through 
which  the  sun  moves,  coincided  with  the  constellations 
marking  the  signs  of  the  zodiac.  But  the  vernal  equi- 
nox, the  point  where  the  sun  crosses  the  equator  in  the 
spring,  moves  very  slowly  backward,  so  that  now  the 
sun  comes  to  the  vernal  equinox  about  a  month  before 
it  enters  the  constellation  Aries.  The  sun,  therefore, 
is  in  the  sign  Aries  while  it  is  in  the  constellation  Pisces, 
and  in  the  sign  Taurus  while  in  the  constellation  Aries, 
etc.  The  signs  of  the  ecliptic  are  about  one  place  ahead 
of  the  corresponding  signs  and  constellations  of  the 
zodiac. 

Although  the  planets  all  move  about  the  sun  in  the 
same  direction,  yet  their  orbits  do  not  lie  in  the  same 
plane.  But  the  angles  which  the  planes  of  the  orbits 
make  with  each  other  are  all  small,  and  the  planets  are 
always  found  within  the  zodiac.  Their  paths  are  ap- 
parently circles,  cutting  the  ecliptic  at  two  points  180 
degrees  apart.  These  points  are  called  nodes.  Since 
the  planets  are  always  so  close  to  the  ecliptic,  when- 
ever they  can  be  seen  they  show  us  just  about  where 
the  ecliptic  lies  in  the  sky. 

1  From  the  Latin  Zoon,  an  animal.  So  named  from  the  animals 
with  which  the  ancients  supposed  it  peopled.  See  page  43. 


44  ASTRONOMY. 


CHAPTER    II 

THE  SUN.      © 

Distance  from  the  Earth,  93,000,000  Miles.1  Diameter, 
866,000  Miles.  Axial  Rotation,  25  Days.  Specific  Grav- 
ity, 1.4. 

33.  The  Sun's  Parallax. — In  finding  the  distance  from 
the  sun  to  the  earth,  astronomers  have  generally  tried 
to  determine  first  the  sun's  parallax.2  The  parallax  of 
a  heavenly  body  is  the  angle  that  the  earth's  radius 
would  make  if  seen  from  that  body.  And  so  the  sun's 
parallax  is  the  angle  that  the  earth's  radius  of  nearly 
four  thousand  miles  would  make,  or,  more  properly 
speaking,  would  subtend,  if  looked  at  from  the  sun. 


FIG.  10.— THE  SUN'S  PARALLAX  (grefltly  exaggerated). 

Fig.  10  will  make  this  clear.     E  is  the  centre  of  the 
earth,  and  AE  is  the  earth's  radius.     Then,  if  S  repre- 


1  In  kilometres,  now  so  frequently  used  for  scientific  measurements, 
the  sun's  distance  is  between  149  and  150  millions.     A  kilometre  is 
nearly  two-thirds  of  a  mile. 

2  Par/al-lax,  from  a  Greek  word  spelled  almost  exactly  the  same 
way,  and  having  the  same  meaning. 


THE  SUN.  45 


sents  the  sun,  the  angle  ASE  is  the  sun's  parallax.1  An 
accurate  measurement  of  the  sun's  parallax  is  exceed- 
ingly difficult,  but  so  great  is  its  importance  that  many 
efforts  have  been  made  to  determine  it.  Some  of  the 
most  successful  methods  will  be  explained  later  in  the 
book,  Arts.  56,  57.  It  is  a  very  small  angle;  the 
best  measurement  so  far  makes  it  8.81".2  The  angle 
at  S  in  Fig.  10  is  greatly  exaggerated ;  it  is  almost 
three  thousand  times  as  large  as  the  real  angle.  To 
represent  it  exactly  in  a  figure  is  of  course  impossible. 
It  is  the  angle  which  a  foot-rule  would  subtend  at  a 
distance  of  four  and  a  half  miles. 

34.  Distance  and  Size  of  the  Sun. — Since  the  earth's 
radius  is  known  very  exactly  (Art.  65),  when  we  know 
the  angle  that  it  subtends  at  the  sun,  it  is  an  easy 
problem  in  trigonometry  to  calculate  the  distance 
of  the  sun,3  which  will  be  found  to  be  a  little  less  than 
93,000,000  miles.  This  distance  has  been  aptly  called 
the  yard-stick  of  the  universe.  Our  measurements  of 
the  distances  and  dimensions  of  all  the  other  planets, 


1  Properly  speaking,  this  is  the  horizontal  parallax, — that  is,  the 
angle  subtended  by  the  radius  running  to  our  feet  when  the  sun  is  on 
the  horizon.     It  is  easily  seen  that  if  the  sun  were  above  its  position 
in  Fig.  10,  the  angle  ASE  would  be  smaller.     And  if  the  sun  were 
directly  above  A,  this  angle  would  be  zero. 

2  A  few  of  our  readers  may  need  to  be  reminded  that  this  is  8.81 
seconds,  and  is  angular  measure.     It  must  not  be  confounded  with 
seconds  of  time,  which  are  never  indicated  by  these  two  strokes  //t 
but  always  by  s,  or  sec. 

8  The  following  proportion  will  make  this  clear  to  those  who  under- 
stand trigonometry.  Using  the  triangle  in  Fig.  10,  in  which  A  is  a 
right  angle  and  ASE  is  the  parallax,  we  have  : 

Sin  parallax  :  sin  90°  :  :  earth's  radius  :  dist.  from  sun  to  earth  ? 
or,  sin  8.81"  :  sin  90°  :  :  3959  :  required  distance. 


46  ASTRONOMY. 


and  even  of  the  distances  of  the  fixed  stars,  depend 
upon  it.  If  the  distance  to  the  sun  is  determined 
more  accurately,  all  these  distances  and  dimensions  as 
given  in  this  book  should  be  proportionately  changed. 
On  this  account  these  figures  will  be  found  to  differ 
in  different  astronomies. 

By  measuring  the  apparent  angular  diameter1  of  the 
sun,  and  knowing  its  distance  from  us,  another  simple 
trigonometrical  solution  gives  us  its  diameter,2  which 
is  about  109  times  the  earth's  diameter.  And  since 
the  volumes  of  spheres  are  as  the  cubes  of  their  diam- 
eters, the  sun's  volume  is  1093,  or  about  1,300,000  times 
that  of  the  earth.  But  the  density  of  the  sun  is  only 
about  one-fourth  of  the  earth's  density,  so  that  while 
it  would  take  1,300,000  worlds  as  large  as  ours  to  make 
one  as  large  as  the  sun,  yet  it  would  only  take  one-fourth 
of  this  number,  or  about  325,000,  to  make  one  as  heavy 
as  the  sun.  The  force  of  gravity  upon  the  sun  is  much 
greater  than  upon  the  earth,  and,  as  the  weight  of 
a  body  depends  upon  gravity,  anything  would  weigh 
nearly  twenty-eight  times  as  much  upon  the  sun  as  upon 
the  earth.  A  man  who  weighs  one  hundred  and  fifty 
pounds  here  would  weigh  more  than  two  tons  upon 
the  sun,  and  would  be  crushed  to  death  by  his  own 
weight. 

THE  SUN  AND  HIS  SURROUNDINGS. 

35.   The  Sun's  Outer  Atmosphere. — If  it  were  possible 

1  The  angular  diameter  of  the  sun  is  the  angle  which  its  diameter 
subtends  as  seen  from  the  earth. 

2  If  a  right-angled  triangle  be  drawn,  having  the  line  from   the 
centre  of  the  earth  to  the  centre  of  the  sun  as  its  hypothenuse,  and  its 
right  angle  at  the  surface  of  the  sun  (because  the  line  along  which 
the  edge  of  the  sun  is  seen  is  a  tangent),  we  have : 

Sin  90°  :  sin  of  half  of  sun's  angle  :  :  93,000,000  :  sun's  radius. 


THE  SUN.  47 


to  visit  the  sun,  one  would  first  enter  the  corona?  a 
very  light  atmosphere  extending  several  hundred  thou- 
sands of  miles  on  all  sides.  It  is  never  seen  except 
during  a  total  eclipse,  and  then  is  a  bright  cloud-like 
circle  of  light  surrounding  the  darkened  sun.  A  great 
part  of  the  corona  is  made  up  of  streamers  of  light 
extending  from  the  sun  in  various  directions.  Some- 
times these  streamers  stretch  away  in  two  opposite 
directions  only;  often  they  project  in  four  directions, 
giving  the  corona  a  four-sided  appearance.  At  the 
eclipse  of  1878  these  streamers  were  noticed  by  some 
observers  to  extend  as  far  as  9,000,000  of  miles  from 
the  sun.  The  corona  is  never  twice  of  the  same  shape, 
and  even  during  the  same  eclipse  its  shape  appears 
very  different  to  different  observers.2  Photographs  of 
it  taken  from  different  points  on  the  earth  at  about 
the  same  time  will,  however,  show  the  same  general 
features.  Fig.  11  represents  a  sketch  of  the  corona  as 
seen  by  Prof.  Stone3  during  the  eclipse  of  1878. 

The  spectroscope  (Art.  254)  shows  that  the  corona 
is  composed  mostly  of  hydrogen,  which  is  the  lightest 
known  gas  upon  the  earth,  and  some  unknown  gas  or 
vapor  even  lighter  than  hydrogen,  which  has  been 

1  Cor-o'na,  Latin  corona,  a  crown. 

2  This  is  remarkable.     Different  observers  of  the  same  eclipse,  even 
when  sitting  side  by  side,  make  totally  different  drawings  of  the  same 
corona.     This  is  probably  because  one  observer's  attention  is  attracted 
mainly  or  even  only  by  those  features  of  the  corona  which  strike  him 
as  most  prominent, — perhaps  the  great  length  or  breadth  of  certain 
streamers.     Another  might  notice  particularly,  and  therefore  draw 
only,  the  brighter  parts  of  the  corona.     And  owing  to  the  short  time 
that  the  eclipse  lasts  and  to  the  excitement  of  the  observers,  probably 
none  of  them  will  notice  all  the  parts  of  the  corona. 

30rmond  Stone.  1847 — ,  director  of  the  Observatory  of  the  Univer- 
sity of  Virginia. 


48  ASTRONOMY. 


called  coronium.  These  gases  give  out  light  of  them- 
selves, and  not  merely  reflect  the  sunlight.  They  are 
exceedingly  thin  and  rare,  more  so  than  our  own  atmo- 


FIG.  11. — THE  CORONA  AS  SEEN  IN  1878. 

sphere,  and  the  streamers  are  probably  more  like  the 
streaks  of  "Northern  Lights"  than  anything  else  we 
know  of  on  the  earth.  They  also  have  a  certain  re- 
semblance to  the  tails  of  comets,  and  may  owe  their 
origin  to  electrical  action. 

36.  The  Sun's  Lower  Atmosphere. — The  lower  part  of 
the  sun's  atmosphere,  which  rests  directly  upon  the 
sun  itself,  is  called  the  chromosphere.1  It  is  a  sheet  of 
flame  several  thousands  of  miles  deep  surrounding  the 
sun.  The  spectroscope  shows  that  the  chromosphere  is  made 
up  of  the  burning  vapors  of  iron,  copper,  sodium,  and  some 

1  ChrO'mo-sphere,  from  the  Greek  chrdma,  color,  and  sphere.  It 
Us  this  layer  of  burning  vapors  that  causes  the  dark  lines  in  the  sun's 
spectrum,  as  is  explained  in  Art.  263. 


THE  SUN. 


49 


twenty  or  more  other  substances  ivhich  we  find  upon  ike  earth. 
Besides  these,  there  are  several  substances  burning 
in  the  chromosphere  which  have  never  been  found 
upon  the  earth.  This  discovery  of  the  substances 
which  compose  the  chromosphere  is  one  of  the  most 
remarkable  of  modern  times.  It  was  made  by  Prof. 
G.  R.  Kirchoif  (kirk'hof ),  of  Germany,  in  1859.  The 
chromosphere  cannot  be  seen  with  the  naked  eye,  nor 
with  an  ordinary  telescope,  except  during  a  total  eclipse 
of  the  sun.  But  by  having  a  spectroscope  attached  to  a 
telescope  (Art.  254),  and  directing  it  to  the  edge  of  the 
sun,  the  chromosphere  can  be  observed  on  any  clear  day. 
37.  The  Solar  Prominences. — Terrible  storms  are  con- 
stantly raging  in  the  chromosphere.  From  every  part 


FIG.  12. — CHANGES  IN  A  SUN  PROMINENCE  DURING  TEN  MINUTES,  OBSERVED  BY  PROFESSOR 
YOUNG,  OCTOBER.  7, 1869. 

of  the  sun's  surface  great  masses  of  the  burning  vapors 
are  frequently  hurled  up  to  a  height  which  not  uncom- 
monly reaches  100,000  miles.  Prof.  Young,  in  1880, 

5. 


50  ASTRONOMY. 


saw  one  thrown  up  to  the  enormous  height  of  350,000 
miles.  These  are  the  red  prominences  seen  during 
total  eclipses  of  the  sun,  and  now,  with  the  aid  of  the 
spectroscope,  watched  every  day.  These  masses  are 
frequently  thrown  up  with  a  velocity  of  100  miles,  and 
sometimes  even  200  miles,  per  second.  They  are  largely 
composed  of  burning  or  glowing  hydrogen,  but  some- 
times, near  the  base,  of  the  burning  vapors  of  the 
metals  and  heavy  elements  which  make  up  the  sun. 
They  must  be  caused  by  great  eruptions  or  explosions 
in  the  sun  or  the  chromosphere.  Fig.  12  shows  the 
sudden  changes  in  one  of  these  prominences,  as  seen 
by  Prof.  Young1  in  1869.  Others  of  the  prominences 
remain  unchanged  in  form  and  position  for  days. 
These  may  be  great  masses  of  clouds  thrown  up  by 
an  explosion,  which  remain  floating  in  the  sun's  at- 
mosphere. 

38.  The  Surface  and  Interior  of  the  Sun. — Below  the 
corona  and  chromosphere  we  come  to  the  surface  of 
the  sun  itself,  the  only  part  of  it  ever  seen  by  most 
people,  called  by  astronomers  the  photosphere.2  This  is 
now  generally  believed  to  be  a  shell  of  clouds  surround- 
ing the  unseen  mass  of  the  sun  beneath.  Every  one 
knows  that  the  clouds  about  the  earth  are  made  up  of 
tiny  drops  of  water,  that  clouds  are  in  fact  precisely 
like  fogs,  except  that  they  are  floating  high  up  in  the 
air.  The  clouds  which  make  up  the  sun's  surface  are 
not  composed  of  water,  but  of  tiny  drops  of  fiery-hot 
melted  iron,  copper,  and  other  substances  that  consti- 
tute the  chromosphere. 

1  Charles  A.  Young,  1834 — ,  Professor  of  Astronomy  at  Princeton, 
New  Jersey. 
*  ^fcO'to-sphere,  from  Greek  phos,  light,  and  sphere. 


THE  SUN.  51 


Within  the  photosphere  is  the  body  of  the  sun,  and, 
strange  as  it  may  seem,  it  is  now  generally  "believed 
that  this  is  a  great  hall  of  gas ;  in  fact,  an  enormous 
bubble.  The  great  pressure  makes  this  gas  denser 
than  water,  so  that  it  is  not  light  and  thin  like  the  air 
around  us,  but  probably  as  thick  as  tar  or  jelly.  This 
gas  is  no  doubt  composed  of  the  vapors  of  the  various 
substances  which  make  up  the  chromosphere.  These 
are  all  kept  in  the  condition  of  vapor  by  the  intense 
heat. 

39.  Sun-spots. — With  a  small  telescope  the  only  thing 
to  be  seen  on  the  sun's  surface  is  a  greater  or  less 


Fio.  13. — SUN-SPOTS  AND  FACUI-;E.     (From  Young's  The  Sun.) 

number  of  dark  spots.  The  shapes  of  these  are  very 
various  and  irregular.  The  central  part  of  a  spot, 
called  the  nucleus,  or  umbra*  is  black,  while  around  the 

1  Um/bra,  Latin  umbra,  a  shadow. 


52  ASTRONOMY. 


edge  is  a  lighter,  grayish  border,  the  penumbra.1  Fig. 
14,  a  drawing  of  a  sun-spot  seen  through  a  large  tele- 
scope in  1860,  shows  very  clearly  the  features  of  a 
sun-spot.  Here  filaments  of  the  penumbra  stretch 
entirely  across  the  umbra ;  but  this  is  unusual.  These 
spots  are  of  all  sizes,  from  those  just  visible  in  large 
telescopes  to  occasional  monstrous  ones  100,000  miles 
in  diameter.  They  are  very  commonly  found  in  groups, 
and  are  not  distributed  over  the  whole  surface  of  the 
sun,  but  are  confined  to  two  zones,  one  on  each  side 
of  the  equator.  These  zones  begin  about  10°  from  the 
equator,  and  end  about  35°  from  it.  Close  to  the  sun's 
equator  spots  are  rarely  seen,  and  close  to  the  poles, 
never.  As  the  sun  turns  upon  its  axis,  the  spots  are 
carried  along  with  it,  and  so  pass  across  the  sun's 
disk  in  twelve  or  fourteen  days ;  it  is  by  the  motion 
of  the  spots  that  we  can  tell  that  the  sun  rotates,  and 
determine  the  time  of  its  rotation.  Besides  being  thus 
carried  around  by  the  sun,  the  spots  have  some  motion 
of  their  own  over  the  sun's  surface.  Careful  observa- 
tions have  shown  also  that  the  spots  in  different  lati- 
tudes have  different  rates  of  rotation.  Spots  on  the 
equator  revolve  in  tw<mty-five  days,  those  farthest  from 
the  equator  in  twenty-six  or  twenty-seven  days.  This 
remarkable  fact  has  made  it  very  difficult  to  decide 
what  the  period  of  the  sun's  rotation  really  is,  but,  as 
Prof.  Young  says,  "the  probability  is  that  the  sun, 
not  being  solid,  has  really  no  exact  period  of  rotation, 
but  different  portions  of  its  surface  and  of  its  internal 
mass  move  at  different  rates,  and  to  some  extent  inde- 
pendently of  each  other." 

1  Pe-num/bra,  Latin  pene,  almost,  and  umbra,  shadow. 


THE  SUN. 


53 


Fio.  14. — A  GROUP  OF  SUM-SPOTS.    (From  Young's  The  Sun.) 


54  ASTRONOMY. 


40.  Phenomena  and  Cause  of  Sun-spots. — The  spots  are 
certainly  great  cavities  in  the  surface  of  the  sun,  the 
bottom  of  the  cavity  forming  the  umbra,  and  the  sides 


Fio.  15.— THE  CHANGES  IN  THE  APPEARANCE  OF  A  SUN-SPOT  AS  IT  is  CARRIED 
ACROSS  THE  SUN'S  DISK  BY  THE  ROTATION  OF  THE  SUN.  (From  Newcomb's  Pop- 
ular Astronomy.) 

the  penumbra.  This  is  shown  by  the  appearance  of  a 
spot  as  it  is  first  brought  into  view  by  the  revolution 
of  the  sun.  This  may  be  seen  in  Fig.  15.  When  the 
spot  is  first  seen  on  the  edge  of  the  sun,  the  penumbra 
and  side  of  the  umbra  nearest  to  us  would  be  hidden, 


THE  SUN.  55 


but  as  the  sun  turned  the  whole  spot  would  pres- 
ently be  seen.  In  going  off  the  sun,  the  other  side  is 
hidden  first.  That  the  sun's  spots  are  cavities  is  also 
conclusively  proved  by  the  fact  that  when  just  upon  the 
edge  of  the  sun  they  have  sometimes  been  seen  to  be 
notches.  The  umbra  is  not  entirely  dark,  but  only  so 
much  darker  than  the  brilliant  surface  of  the  photo- 
sphere as  to  look  dark  when  compared  with  it.  The 
highest  artificial  light  that  can  be  made,  except  the 
electric  light,  seems  absolutely  black  compared  with  the 
sun's  light.  Spots  may  last  for  months,  or  only  for 
hours.  They  appear  and  disappear  with  great  rapidity, 
and  frequently  change  their  size  and  appearance  greatly 
from  day  to  day.  A  large  spot  frequently  breaks  up 
into  small  ones,  and  a  group  of  small  ones  as  frequently 
combine  to  make  a  large  one. 

The  cause  of  the  sun-spots  is  another  of  the  mysteries 
of  this  wonderful  body.  As  has  been  said,  they  are 
certainly  great  hollows  or  cavities,  and  may  be  caused 
by  great  whirlwinds,  just  as  we  see  whirlpools  formed 
in  water.  And  there  is  evidence  that  these  cavities  are 
filled  with  gases  and  vapors,  which  obstruct  the  light 
from  below,  and  so  cause  the  dark  parts  of  the  spot; 
that  they  are  places  where  the  gases,  which  have  been 
forced  up  in  the  prominences,  and  cooled  in  the  upper 
layers  of  the  atmosphere,  are  again  drawn  down  into 
the  sun. 

41.  How  to  Observe  the  Sun-spots. — When  the  spots 
are  very  large,  they  can  be  seen  by  the  naked  eye, 
looking,  of  course,  through  smoked  or  colored  glass ; 
but  this  is  uncommon.  With  any  good  spy-glass  they 
can  generally  be  seen.  If  they  are  observed  directly 
through  a  spy-glass  or  a  telescope,  the  eye-piece  must 


56  ASTRONOMY. 


always  be  covered  with  a  dark  glass.  Several  astrono- 
mers have  lost  their  eyesight  by  looking  at  the  sun 
through  a  telescope  without  using  the  colored  shade. 
Unless  a  small  glass,  or  very  low  magnifying  power,  is 
used,  one  must  not  expect  to  see  the  whole  sun  at 
once ;  the  instrument  must  be  moved  gently  over  the 
surface  to  scan  it  all.  It  may  be  well,  too,  to  remind 
the  young  observer  that  an  astronomical  telescope  al- 
ways inverts  the  object  seen :  what  seems  in  the  tele- 
scope to  be  the  lower  part  of  the  sun  is  really  the  upper 
part,  what  seems  to  be  the  right  side  is  the  left  side. 
But  a  spy-glass  does  not  invert  the  object.1 

A  better  way  to  observe  the  sun-spots  is  to  throw 
the  sun's  image  upon  a  screen  with  the  spy-glass  or 
telescope.  For  this  a  room  having  a  window  towards 
the  sun  must  be  chosen,  and  it  must  be  darkened  with 
shutters  or  curtains.  The  instrument  is  then  pointed 
through  a  hole  in  the  shutter  or  curtain  at  the  sun,  just 
as  if  it  was  to  be  observed  in  the  ordinary  way.  But 
instead  of  looking  at  the  sun,  place  a  screen  or  piece 
of  white  paper  perpendicular  to  the  telescope  and  a 
short  distance,  say  a  foot,  from  the  eye-piece,  when  a 
brilliant  image  of  the  sun  will  be  seen  upon  the  screen. 
The  instrument  ought  to  be  upon  a  stand,  or  supported 
by  some  fixture  attached  to  the  window  or  shutter,  and 
may  be  directed  to  the  sun  by  glancing  along  the  top 
of  the  tube.  When  the  image  is  once  thrown  upon 
the  screen,  it  can  easily  be  kept  there  by  gently  moving 
the  telescope  as  the  image  passes  off.  The  instrument 
must  be  focused  by  moving  the  eye-piece  in  or  out 
until  the  picture  is  most  distinct.  The  whole  of  the 
sun  will  not  generally  be  shown  at  once,  but  by  moving 

1  The  cause  of  this  is  explained  in  Art,.  244. 


THE  SUN.  57 


the  instrument  all  the  different  parts  of  its  surface  may 
be  thrown  upon  the  screen  and  examined.  No  dark 
glass  is  needed  to  cover  the  eye-piece,  and  the  sun's 
image  with  its  spots  may  be  seen  by  a  number  of  per- 
sons ait  once.  The  image  may  be  made  as  large  as  is 
wished  by  moving  the  screen  farther  off,  or  throwing 
the  picture  upon  the  opposite  wall,  but  the  smaller 
images  will  be  most  brilliant.  By  this  method  all  the 
phenomena  and  changes  of  the  spots  may  be  carefully 
studied,  their  motions  and  changes  noted,  and  their 
outlines  drawn  upon  the  screen  itself.  If  an  astro- 
nomical telescope  is  used,  the  observer  will  not  forget 
that  the  motions  of  the  spots  are  just  opposite  to  their 
apparent  motions  from  day  to  day  upon  his  screen,  and 
to  give  them  their  correct  positions  his  drawings  must 
be  turned  upside  down.  The  general  motion  of  the 
spots  is  from  east  to  west,  or,  when  looking  at  the 
sun,  from  left  to  right,  but  not  directly  across;  the 
direction  of  the  motion  shows  that  the  sun's  axis  is 
inclined  to  the  plane  of  the  earth's  orbit.  No  heavenly 
body  can  be  observed  or  studied  with  more  interest  by 
the  owner  of  a  spy-glass  or  small  telescope  than  the  sun. 
Accurate  and  complete  records  of  sun-spots,  accom- 
panied if  possible  by  drawings,  would  be  valuable  con- 
tributions to  science,  and  many  important  discoveries 
have  been  made  in  this  field  with  small  instruments. 

42.  Periodicity  of  the  Spots. — Long  observations  have 
proved  the  curious  fact  that  sun-spots  are  most  abun- 
dant about  every  eleven  years.  In  1848,  1860,  and 
1870  they  were  most  numerous,  while  in  1856, 
1867,  and  1878  they  were  fewest.  In  1882  and  1883 
they  were  abundant,  especially  in  the  latter  year; 
then  they  diminished  in  size  and  frequency  until 


58  ASTRONOMY. 


1889,  increasing  again  until  the  next  maximum  in 
1893.  No  satisfactory  cause  of  this  periodicity  has 
been  discovered.  Observation  has  also  shown  a  con- 
nection between  the  sun-spots  and  magnetic  disturb- 
ances upon  the  earth.  When  sun-spots  are  most 
abundant,  magnetic  storms  are  most  frequent ;  that  is, 
compass-needles  are  turned  from  their  proper  direction, 
strong  magnetic  currents  take  possession  of  the  tele- 
graph wires,  interfere  with  the  sending  of  messages, 
and  even  set  telegraph-offices  on  fire.  These  magnetic 
storms  may  be  noticed  over  the  whole  earth,  and  are 
sometimes  accompanied  by  unusual  displays  of  the 
aurora,  or  northern  lights.  Like  the  sun-spots,  these 
phenomena  are  periodical,  and  their  periods  coincide 
with  the  sun-spot  periods.  The  cause  of  this  coinci- 
dence is  unknown,  but  there  is  probably  an  electrical 
connection  between  the  sun  and  the  earth.  All  of 
these  phenomena  are  well  worth  observing  and  noting 
down.  Such  observations  may  lead  to  valuable  results. 

43.  Other  Markings  on  the  Sun. — Seen  through  a  good 
telescope,  the  whole  bright  surface  of  the  sun  is  mot- 
tled, being  covered  apparently  by  bodies  which  from 
their  shape  and  appearance  have  been  called  rice-grains. 
"Perhaps  the  most  familiar  idea  of  this  appearance 
will  be  presented  by  saying  that  the  sun  looks  like  a 
plate  of  rice-soup,  the  grains  of  rice,  however,  being 
really  hundreds  of  miles  in  length."1  Under  very 
favorable  circumstances  these  rice -grains  have  been 
seen  to  be  made  up  of  smaller  granules. 

About  twenty  years  ago,  Mr.  JSTasmyth,  an  English 
astronomer,  announced  the  discovery  that  through  a 

1  Newcomb's  Popular  Astronomy,  p.  237. 


THE  SUN.  59 


powerful  telescope  this  mottled  appearance  of  the  sun 
was  seen  to  be  due  to  a  mass  of  long  narrow  bodies 
intertwined  into  a  complete  net-work  over  the  whole 
surface  of  the  sun.  These  did  not  seem  to  him  to  be 
shaped  like  rice-grains,  but  like  willow-leaves.  These 
willow-leaves,  as  they  were  called,  have  not  been  seen 
by  other  observers,  and  their  existence  is  doubtful. 
Fig.  14  was  drawn  by  Mr.  Nasmyth,  and  shows  around 
the  spot  the  appearance  resembling  willow-leaves  that 
he  thought  he  saw. 

Bright  streaks  are  often  seen  upon  the  sun,  some- 
times separate,  and  sometimes  forming  a  net-work. 
These  are  called  faculce.1  They  are  temporary  ridges 
on  the  surface  of  the  sun ;  this  is  proved  by  the  fact 
that  they  have  been  seen  to  project  above  the  edge  of 
the  sun.  They  are  sometimes  many  thousands  of  miles 
long.  They  are  abundant  about  the  edges  of  the  sun- 
spots,  and,  like  the  spots,  they  are  constantly  appearing, 
disappearing,  and  changing  their  forms.  In  years 
when  sun-spots  are  few  faculse  are  few  also.  They 
seem  to  be  heaped  up  by  the  great  storms  and  other 
commotions  on  the  sun,  especially  when  a  sun-spot  is 
formed  or  disappears.  The  white  cloud-like  patches 
shown  in  Fig.  13  are  the  faculse.  The  faculse,  as  well 
as  the  rest  of  the  sun,  are  made  up  of  the  rice-grains. 
These  faculse  can  be  seen  with  a  telescope  of  mod- 
erate size,  and  may  be  observed  directly  or  thrown 
upon  a  screen  as  the  sun-spots  were.  They  should  be 
looked  for  around  spots  which  are  near  the  edge  of 
the  sun.  The  mottled  appearance  of  the  sun  may  be 
seen  in  the  same  ways,  but  needs  at  least  a  good 

1  Fac'u-lse,  plural  of  Latin  facula,  a  torch. 


60  ASTRONOMY. 


three-inch  telescope'  and  careful  observation.  The 
separate  rice-grains  can  be  seen  only  through  large 
telescopes. 

44.  The  Sun's  Position  and  Importance  in  the  Solar 
System. — The  sun  is  the  centre  of  the  solar  system,  and 
his  mass  is  700  times  as  great  as  that  of  all  the  other 
bodies  in  the  system  put  together.     On  account  of  his 
overwhelming  size,  his  great  attraction  controls  the  mo- 
tions of  all  the  planets,  and  keeps  them  in  their  orbits. 
Were  this  attractive  force  of  the  sun  to  cease,  the 
whole  system  would  at  once  go  to  pieces,  the  planets 
would  fly  off  into  boundless  space,  and  all  life  upon 
the  earth  or  elsewhere  in  the  system  would  speedily  be 
destroyed. 

45.  The  Sun's  Light.     Its  Amount  and  Importance. — 
The  sun's  light  is  the  most  intense  light  known  to  us. 
It  is  from  three  to  four  times  as  bright  as  the  brightest 
electric  light;    and  every  other  artificial  light  seems 
absolutely  black  when  put  in  front  of  the  sun.     Several 
attempts  have  been  made  to  measure  the  brightness  of 
the  sun's  light.    This  can  be  done  only  by  comparing  its 
light  with  some  other  light.     For  instance,  it  has  been 
found  that  the  sun  gives  out  600,000  times  as  much 
light  as  the   full   moon,  while  the  light  of  the  full 
moon  is  about  the  same  as  that  of  a  candle  twelve 
feet  away.     Much  of  the  sun's  light  is  absorbed  by  the 
atmosphere  of  the  sun,  and  some  by  the  atmosphere  of 
of  the  earth.     Were  these  removed,  Professor  Langley 
has  calculated  that  the  sun  would  be  two  or  three  times 
as  bright  as  now,  and  blue  instead  of  yellow. 


1  The  size  of  a  telescope  is  generally  designated  by  the  diameter  of 
its  object-glass. 


THE  SUN.  61 


Of  the  importance  of  the  sun's  light  it  is  scarcely 
necessary  to  speak.  Without  it  we  should  have  only 
starlight  in  addition  to  our  artificial  light  ;  for  the 
moon  shines  wholly  by  reflected  sunlight.  Besides  its 
great  importance  in  vision,  sunlight  is  essential  to 
vegetable  life,  and  indirectly,  therefore,  if  not  directly, 
to  all  animal  life. 

46.  The  Sun's  Heat.  —  Its  Amount  and  Importance.  — 
The  amount  of  heat  which  the  sun  gives  out  is  beyond 
all  our  conception.  That  which  the  earth  receives 
every  year  would  melt  a  shell  of  ice  about  165 
feet  deep  covering  the  whole  earth.  Yet  this  is 
l  of  all  that  the  sun  sends  out.  As 


2,300.000.000 

Proctor  puts  it,  "In  each  second  the  sun  gives  out 
as  much  heat  as  would  be  given  out  by  the  burning 
of  11,000,000,000,000,000  tons  of  coal." 

Without  the  sun's  heat  the  temperature  of  the  earth 
would  be  some  hundreds  of  degrees  below  zero,  a  tem- 
perature at  which  it  would  be  impossible  for  life  to 
exist.  But  this  is  not  all,  for  to  the  sun's  heat  almost 
all  motions  on  the  earth  are  due.  All  the  winds,  all 
the  clouds  and  storms,  and  consequently  all  springs  and 
rivers,  are  due  to  the  sun's  heat.  All  our  wood  and 
coal  represent  just  so  much  of  the  sun's  heat  stored  up 
in  the  past.  And,  since  it  is  now  known  that  one  sort 
of  force  may  be  changed  into  another,  the  sun's  heat 
must  be  considered  the  real  cause  of  almost  all  the 
forces,  of  all  the  work,  and  of  all  the  power  in  the 
world.  The  tides  are  perhaps  the  only  exception,  for 
they  are  due  mainly  to  the  moon's  attraction.  But  it 
is  the  sun's  heat  alone  that  keeps  the  water  in  a  liquid 

1  How  is  this  calculated  ? 
6 


62  ASTRONOMY. 

state,  and  thus  allows  it  to  form  tides.  Tyndall  well 
says,  "  The  natural  philosopher  of  to-day  may  dwell 
amid  conceptions  which  heggar  those  of  Milton.  Look 
at  the  integrated  energies  of  our  world, — the  stored 
power  of  our  coal-fields;  our  winds  and  rivers;  our 
fleets,  armies,  and  guns.  What  are  they  ?  They  are 
all  generated  hy  a  portion  of  the  sun's  energy,  which 
does  not  amount  to  2.300,000,000  of  the  whole.  This  is 
the  entire  fraction  of  the  sun's  force  intercepted  by 
the  earth,  and  we  convert  hut  a  small  fraction  of  this 
fraction  into  mechanical  energy.  Multiplying  all  our 
powers  by  millions  of  millions,  we  do  not  reach  the 
sun's  expenditure.  And  still,  notwithstanding  this 
enormous  drain,  in  the  lapse  of  human  history  we  are 
unable  to  detect  a  diminution  of  his  store.  Measured 
by  our  largest  terrestrial  standards,  such  a  reservoir  of 
power  is  infinite ;  but  it  is  our  privilege  to  rise  above 
these  standards,  and  to  regard  the  sun  himself  as  a 
mere  speck  in  its  finite  extension,  a  mere  drop  in  the 
universal  sea." 

47.  The  Cause  of  the  Sun's  Heat. — The  amount  of  heat 
given  off  from  the  sun  continually  is  so  enormous  that, 
as  none  of  this  comes  back,  it  has  been  a  great  problem 
to  account  for  this  constant  supply  of  heat.  We  know 
that  if  the  whole  sun  were  a  mass  of  solid  coal,  it  would 
burn  out  at  its  present  rate  in  five  thousand  years ;  and 
yet  the  sun  has  lasted  much  longer  than  that,  and,  so 
far  as  we  can  notice,  his  heat  is  not  diminishing  a  par- 
ticle. Two  theories  have  been  advanced  to  account 
for  this.  One  is  the  meteoric  theory.  As  will  be  fully 
explained  in  chapter  VIII,  it  is  known  that  immense 
numbers  of  small  bodies  are  revolving  about  the  sun, 
and  some  of  these  must  be  continually  falling  into  it, 


THE  SUN.  63 


just  as  they  fall  upon  the  earth  and  give  us  our  shoot- 
ing-stars, but  the  number  there  must  be  vastly  greater 
than  here.  Now,  when  one  body  strikes  another,  heat 
is  always  produced,  as  when  a  nail  is  struck  with  a 
hammer.  If  the  striking  body  moves  very  swiftly,  the 
heat  produced  is  very  great.  If  a  combustible  body 
were  to  fall  from,  the  earth  to  the  sun,  its  striking 
would  produce  6000  times  as  much  heat  as  the  burn- 
ing of  the  same  body  could.  And  so  it  has  been 
thought  that  the  sun's  heat  is  kept  up  by  the  striking 
of  these  bodies,  called  meteors,  upon  its  surface.  But 
when  astronomers  came  to  realize  the  prodigious  heat 
of  the  sun,  they  saw  that  although  this  cause  helps,  yet 
alone  it  could  not  be  sufficient  to  supply  the  sun  with 
heat.  The  other  theory  of  the  sun's  heat  is  the  contrac- 
tion theory.  It  supposes  that  by  its  own  attraction  the 
sun  is  slowly  contracting  in  bulk :  this  condensation 
or  squeezing  together  would  produce  heat  just  as  a 
body  falling  upon  it  would.  It  has  been  estimated 
that  if  the  sun's  diameter  should  shorten  only  six 
miles  in  one  hundred  years,  as  much  heat  would  be 
produced  as  the  sun  gives  out  in  that  time.  No  such 
contraction  has  ever  been  noticed  in  the  sun,  but  this 
is  no  reason  why  the  theory  may  not  be  true,  for  if  the 
shrinking  has  occurred,  we  could  not  possibly  detect  it 
yet.  This  is  the  only  cause  ever  suggested  that,  so  far 
as  we  now  know,  can  be  the  true  one ;  and,  although  it 
has  not  been  proved,  it  is  generally  regarded  by  as- 
tronomers as  the  principal  cause  of  the  sun's  heat. 

48.  The  Sun's  Past  and  Future. — There  has  been  of 
late  much  speculation  upon  the  probable  length  of  time 
that  the  sun  has  existed,  and  when  he  will  probably 
cease  to  give  out  heat.  No  matter  what  may  be  the 


64  ASTRONOMY. 


source  of  the  sun's  heat,  we  are  forced  to  conclude  that, 
if  natural  laws  alone  operate,  his  heat  must  at  last  be 
exhausted.  As  the  sun  gradually  cooled  off,  the  earth 
would  become  colder  and  colder;  and  when  all  heat 
from  the  sun  ceased,  the  temperature  of  the  earth 
would,  as  has  been  said,  probably  be  hundreds  of  de- 
grees below  zero.  Long  before  this,  time  all  life  would 
of  course  perish  from  the  earth.  But  in  any  event 
these  conjectures  need  give  us  little  immediate  con- 
cern, for  the  rashest  speculators  place  these  events 
millions  of  years  in  the  future. 

4Sa.  The  Sun's  Composition. — The  sun  is  very  largely 
composed  of  the  same  substances  which  exist  on  the 
earth.  The  spectroscope  (pages  298-303)  shows  this 
without  doubt.  Iron,  copper,  calcium,  magnesium, 
sodium,  hydrogen,  and  many  other  materials  are  in 
the  chromosphere  in  a  gaseous  condition.  At  least 
thirty-six  terrestrial  elements  are  known  to  exist  on 
the  sun.  One  of  these,  helium,  was  discovered  on 
the  sun  before  its  existence  on  this  planet  was  sus- 
pected, and  it  was  not  till  1895  that  it  was  found  as 
a  constituent  of  an  earthly  mineral. 


THE  INFERIOR  PLANETS.  65 


CHAPTER    III. 

*• 

THE   INFERIOR   PLANETS. 

49.  Suspected  Vulcan. — For  some  years  certain  as- 
tronomers have  strongly  suspected  that  between  Mercury 
and  the  sun  there  is  a  planet,  which  they  have  named 
Vulcan.1  The  great  French  astronomer  Le  Yerrier,2 
of  whom  we  shall  hear  in  connection  with  the  discov- 
ery of  Neptune,  found  certain  irregularities  in  Mer- 
cury's motion,  which  he  suggested  might  he  caused 
by  the  attraction  of  such  an  inside  planet.  Observers 
have  several  times  announced  that  they  saw  the  planet 
crossing  the  sun's  disk;  but  few,  if  any,  such  obser- 
vations have  been  reported  by  skilled  astronomers, 
and  an  unpractised  observer  might  easily  mistake  a 
sun-spot  for  a  planet.  Besides,  at  the  very  times  when 
some  of  these  supposed  observations  were  made,  other 
and  better  observers  were  also  watching  the  sun,  and 
saw  no  planet.  But  the  strongest  evidence  in  favor  of 
Vulcan's  existence  was  given  in  1878.  During  a  total 
eclipse  of  the  sun  in  that  year,  two  American  astrono- 
mers, Prof.  Watson3  and  Mr.  Swift,4  claimed  to  have 
discovered  two  or  more  planets  within  Mercury's  orbit. 

1  Vul'can,  the  god  of  fire. 

2  L8  Ver'ri-Sr,  1811-1877.     A  great  French  astronomer;  the  dis- 
coverer of  the  planet  Neptune.     See  Art.  179. 

8  James  C.  Watson,  Professor  of  Astronomy  at  Michigan  Univer- 
sity, and  at  University  of  Wisconsin,  1838-1880. 
4  Lewis  Swift,  Astronomer  of  Kochester,  New  York. 

6* 


66  ASTRONOMY. 


Notwithstanding  Prof.  Watson's  great  reputation,  as- 
tronomers generally  seem  to  think  that  he  mistook 
small  stars  for  planets.  No  one  has  been  able  to  find 
these  planets  since,  though  total  eclipses  have  given 
good  opportunities,  and  the  existence  of  Vulcan  must 
be  regarded  as  very  doubtful. 

MERCURY.      5 

Distance  from  Sun,  36,000,000  Miles.    Diameter,  3000  Miles. 
Length  of  Year,  3  Months.    Length  of  Day,  Uncertain. 

50.  Relations  to  the  Solar  System,  and  Features. — So 
far  as  is  certainly  known,  Mercury1  is  the  nearest 
planet  to  the  sun.  Seen  from  him  the  sun  seems 
seven  times  as  large  as  seen  from  the  earth,  and 
upon  his  surface  one  would  get  seven  times  as  much 
light  and  heat  as  upon  the  earth.  Mercury  is  the 
smallest  of  the  eight  principal  planets ;  his  volume  is 
only  ^  of  that  of  the  earth.2  It  is  difficult  to  ascer- 
tain his  mass  or  density,  differing  results  being  ob- 
tained by  different  methods.  The  planet  is  so  close  to 
the  sun  that  observation  of  it  is  very  unsatisfactory. 
In  the  largest  telescopes  its  surface  is  brilliant,  but 
markings  are  seen  with  greatest  difficulty.  Schiaparelli 
of  Milan,  whose  keenness  of  sight  has  often  been 
proved,  thinks  he  has  seen  them,  and  the  evidence 
seems  to  be  that  the  planet  rotates  on  its  axis  in  the 
same  time  that  it  revolves  around  the  sun.  This  would 
keep  the  same  face  continually  turned  towards  the  sun. 
We  shall  presently  see  that  the  moon  moves  about 
the  earth  in  this  way.  Nor  is  it  certainly  known 

1  The  Latin  name  of  the  god  who  acted  as  messenger  for  the  other 
gods.  The  sign  5  represents  his  rod.  All  of  the  principal  planets 
except  the  earth  are  named  for  the  Latin  deities. 

•How  is  this  found? 


THE  INFERIOR  PLANETS.  67 

whether  the  planet  has  an  atmosphere  or  not;  but  it 
is  supposed  to  have  a  very  dense  one. 

51.  Motions  and  Phases. — As  Mercury  is  an  inferior 
planet,  while  revolving  about  the  sun  it  seems  to  us  only 
to  swing  backward  and  forward  past  the  sun  (Art.  24), 
and  is  never  more  than  29°  from  it.  When  opposite  the 
sun  from  the  earth,  it  is  obscured  by  the  sun's  brightness 
and  cannot  be  seen.  As  soon  as  it  is  far  enough  from 
the  sun  to  be  visible,  it  is  small  and  nearly  round.  At 
one  side  of  the  sun,  or  at  its  greatest  elongation  (Art. 
24),  as  at  E  in  Fig.  16,  it  is  larger,  but  only  half  full. 


FIG.  16.— THE  CHANGES  IN  MEKCUBY  AS  IT  BEVOLVES  ABOUND  THE  SUN. 

As  it  comes  around  between  the  sun  and  the  earth  it 
grows  still  larger,  but  is  a  crescent,  growing  narrower 
and  narrower,  until  in  passing  between  us  and  the  sun 
it  is  lost  Li  the  sun's  glare,  unless  it  should  happen 
to  go  directly  across  the  sun,  when  it  could  be  seen 
as  a  black  spot  on  his  face.1  These  varying  phases 
are  just  like  those  of  the  moon,  and  prove  that  the 
planet  shines  by  reflecting  the  sun's  light.  "When 

1  How  far  are  Mercury  and  the  earth  apart  when  they  are  on  oppo- 
site sides  of  the  sun  ?  when  on  the  same  side  ? 


68  ASTRONOMY. 

on  the  opposite  side  of  the  sun,  the  half  of  the  planet 
lighted  up  is  turned  towards  us,  and  it  is  about  full. 
When  the  planet  is  at  one  side  of  the  sun,  we  see  only 
half  of  the  lighted  hemisphere,  and  as  it  comes  more 
and  more  between  us  and  the  sun,  its  bright  side, 
always  being  the  one  towards  the  sun,  is  turned  more 
and  more  away  from  us,  while  the  dark  hemisphere 
is  turned  more  and  more  towards  us,  so  that  only  a 
crescent  of  light  can.  be  seen,  growing  constantly  nar- 
rower, until,  if  the  planet  transits  the  sun,  its  dark  side 
is  entirely  towards  us,  and  it  is  a  round  black  spot.1 

52.  Transits  of  Mercury. — Every  few  years  Mercury 
passes  directly  across  the  sun's  disk.     The  transits  are 
important  in  astronomical  calculations,  but  are  of  little 
interest  to  observers.     So  small  is  the  planet  that  the 
transit  cannot  be  seen  with  the  naked  eye,  but  may  be 
seen  with  a  small  telescope  or  a  spy-glass.    It  is  simply 
a  small  black  spot  on  the  face  of  the  sun,  crossing  it  in 
a  few  hours.     The  next  transits  of  Mercury  will  occur 
on  the  following  dates  : 

November  10,  1894. 
November  4,  1901. 

53.  How  to  observe  Mercury. — Mercury  is  always  near 
the  sun,  and  on  that  account  is  seldom  seen  except  by 
astronomers.     It  can  only  be  seen  with  the  naked  eye 
about  the  time  of  greatest  elongation.     It  may  then 
set  one  and  one-half  hours  after  the  sun,  or  rise  one 

1  It  is  important  that  these  phases  and  their  cause  should  be  clearly 
understood.  If  a  careful  reading  of  the  explanation  does  not  clear 
the  matter  up,  a  diagram,  or  a  representation  of  Mercury's  motion 
by  moving  any  object  around  an  imagined  sun,  and  between  that  and 
an  imagined  earth,  together  with  a  little  study,  will  make  it  clear. 
In  which  direction  from  Fig.  16  is  the  earth  supposed  to  be  ? 


THE  INFERIOR  PLANETS.  69 

and  one-half  hours  before  it,  but  the  twilight  and  its 
nearness  to  the  horizon  interfere  very  much  with  its 
observation.  The  times  of  greatest  elongation  may 
be  found  in  our  common  almanacs.  For  a  week 
or  more  before  and  after  these  days  the  planet 
may  be  looked  for.1  If  at  these  times  a  strange 
star  be  seen  near  the  place  on  the  horizon  where  the 
sun  went  down,  one  may  be  pretty  certain  that  he  has 
found  the  planet.  It  will  not  appear  very  bright,  but 
will  be  as  bright  as  any  fixed  star  would  appear  in  its 
position,  The  beginner  must  not  be  disappointed  if 
he  have  difficulty  in  finding  this  planet,  or  even  if  he 
fail  to  find  it.  The  great  Copernicus  never  succeeded 
in  finding  it ;  but  this  was  largely  due  to  his  northern 
latitude,  where  twilight  lasts  longer.  To  the  naked 
eye  the  planet  looks  just  like  a  star ;  with  a  small  tele- 
scope the  phases  can  be  seen,  which  are  its  only  inter- 
esting features.  When  in  its  most  favorable  position 
for  observation  it  is  always  about  half  full. 


1  In  order  that  astronomical  observation  may  be  most  successful, 
the  body  should  be  observed  when  at  a  considerable  distance  above 
the  horizon  ;  for  any  one  may  notice  that  even  upon  a  very  clear  night 
stars  close  to  the  horizon  cannot  be  seen  well,  if  at  all.  This  direc- 
tion cannot,  however,  often  be  observed  with  Mercury.  In  general, 
moonless  nights  are  the  best  for  astronomical  observation,  although 
in  the  case  of  the  bright  planets  the  moon  will  interfere  but  little 
if  it  is  not  in  their  immediate  neighborhood.  Then,  of  course, 
the  atmosphere  and  sky  must  be  clear.  The  atmosphere,  however,  is 
occasionally  quite  deceptive.  It  will  sometimes  be  very  unfit  for  tele- 
scopic work,  especially  if  high  powers  (Art.  247)  be  used  on  the  tele- 
scope, when  it  seems  to  be  perfectly  clear ;  and  nights  which  seem 
to  be  hazy  may  be  found  to  be  excellent  for  observation.  The  only 
way  to  determine  the  matter  will  be  to  bring  out  the  telescope  and 
try  it 


70  ASTRONOMY. 


VENUS.     ? 

Distance  from  Sun,  67,000,000  Miles.  Diameter,  7600  Miles. 
Length  of  Tear,  7|  Months.  Length  of  Day,  Uncertain. 
Specific  Gravity,  4|. 

54.  Relations  to  the  Solar  System,  and  Description. — Be- 
tween the  orbits  of  Mercury  and  the  earth  is  Venus.1 
She  comes  nearer  to  us  than  any  of  the  other  planets, 
being  sometimes  only  about  25,000,000  miles  away;2 
but  she  gets  twice  as  much  light  and  heat  as  the  earth.3 
Venus  is  almost  the  same  size  as  the  earth,  her  diame- 
ter being  only  three  hundred  miles  less  than  the  earth's. 
Notwithstanding  her  nearness,  this  planet  is  very  dif- 
ficult to  observe.  Some  astronomers  have  announced 
markings  on  its  surface  and  a  blunting  of  its  horns 
when  crescent-shaped,  and  have  thus  deduced  a  period 
of  rotation  of  about  23J  hours;  others  with  equally 
good  opportunities  have  strenuously  denied  the  exist- 
ence of  any  such  markings.  Hence,  while  it  is  prob- 
able that  they  exist,  they  are  very  obscure  and  hard 
to  see;  it  is  not  unlikely  that,  like  Mercury,  it  ro- 
tates on  its  axis  in  the  same  time  it  makes  a  revo- 
lution around  the  sun.  There  is  strong  evidence  of 
a  dense  atmosphere,  and  it  would  seem  that  this 
and  its  probable  thick  clouds  reflect  the  sun's  light 
so  brightly  that  we  never  see  the  surface  of  the 
planet  at  all. 

1  Venus,  the  goddess  of  beauty  and  love.     Her  sign  is  9,  a  mirror. 

a  How  is  this  found  ?  "What  is  her  greatest  distance  from  the  earth  ? 

»  The  amount  of  light  and  heat  that  a  planet  receives  from  the  sun 
depends  upon  the  square  of  its  distance.  The  earth  is  about  1£  times 
as  far  from  the  sun  as  Venus.  The  square  of  1£  is  2J,  or  about  2' 
therefore  the  earth  receives  one-half  as  much  heat  and  light. 


THE  INFERIOR  PLANETS. 


55.  Motions  and  Phases.  —  As  Venus  is  also  an  infe- 
rior planet,  she  swings  from  one  side  of  the  sun  to 
the  other,  and  passes  through  her  phases  just  like 
Mercury,  but  on  a  larger  scale.  At  her  greatest 
elongation  Venus  is  forty-seven  degrees  from  the  sun, 
and,  owing  to  the  great  difference  in  her  distances  from 
us,  her  size  varies  much  more  than  Mercury's. 

Fig.  17  shows  the  appearance  and  comparative  sizes 


Fio.  17  — THE  APPEARANCE  AND  COMPARATIVE  SIZES  OF  VENUS  IN  ITS  DIFFERENT  ?2.«SB& 

of  Venus  when  nearly  between  the  earth  and  the  sun, 
when  at  greatest  elongation,  and  when  on  the  opposite 
side  of  the  sun.1 

56.  Transits  of  Venus. — These  are  much  rarer  phe- 
nomena than  transits  of  Mercury,  and  have  been  con- 
sidered to  be  of  great  importance,  because  they  have 
hitherto  been  thought  to  afford  the  best  opportunity 
of  finding  the  distance  from  the  sun  to  the  earth.  To 
find  this,  stations  are  chosen  on  opposite  sides  of  the 

1  The  shaded  parts  of  the  figure  are  only  intended  to  fill  out  the 
disks.  The  dark  part  of  the  moon  can  sometimes  be  seen,  but  Hot  so 
"with  Venus. 


72  ASTRONOMY. 


earth,  in  the  northern  and  southern  hemispheres,  as  at 
1ST  and  S  in  Fig.  18.  To  the  observer  at  N",  Venus 
seems  to  cross  the  sun  on  the  line  HF ;  to  the  one  at 
S,  it  crosses  higher  up,  on  CD.  Each  observer  deter- 
mines carefully  where  the  planet  seems  to  cross,  and 


Pro.  18.— TRANSIT  OF  VENUS. 

this  gives  the  angular  distance  between  A  and  B. 
This,  with  the  distance  from  !N"  to  S,  determined  on 
the  earth,  and  the  comparative  distances  of  the  earth  and 
Venus  from  the  sun,  which  are  found  by  Kepler's 
third  law  (Art.  29),  will  enable  a  person  who  has  a 
knowledge  of  geometry  and  trigonometry  to  find  the 
distance  to  the  sun.1  The  real  calculation  of  the  dis- 

1  NVS  and  A VB  may  be  taken  to  be  similar  isosceles  triangles  .• 

therefore 

NV  :  VB  :  :  NS  :  AB. 

But,  by  Kepler's  third  law, 

NV  :  YB  :  :  1  :  2.61, 

and  therefore 

1  :  2.61  :  :  NS  :  AB ;  or,  AB  =  2.61NS. 

Suppose  that  NS  is  the  diameter  of  the  earth,  7918  miles,  and  that 
AB  has  been  found  to  be  46".  Then  AB  =  20,666  miles  =  46", 
and  1"  =  449  miles,  which  shows  that  449  miles,  seen  at  the  distance 
between  the  earth  and  the  sun,  subtends  an  angle  of  1".  The  earth's 
radius,  then,  if  seen  from  the  sun,  would  subtend  an  angle  of  |9¥5/ 
seconds,  or  about  8.81",  which  is  the  sun's  parallax  (Art.  33). 
Knowing  the  parallax  and  the  earth's  radius,  the  solution  of  a  right- 
angled  triangle  (see  foot-note  on  page  45)  gives  the  distance  in  miles 
from  the  earth  to  the  sun. 


THE  INFERIOR  PLANETS.  73 


tance  of  the  sun  by  this  method  is  a  very  complicated 
problem.1  The  observations  made  during  the  transit  of 
1769  were  not  completely  worked  up  for  fifty  years,  and 
those  made  in  1874  and  1882  required  years  for  comple- 
tion. When  the  distance  of  the  earth  from  the  sun  is 
found,  the  distances  of  all  the  other  planets  are  easily 
found  by  Kepler's  third  law,  and  their  diameters  can 
then  be  found  just  as  the  sun's  was  found  in  Art.  34. 

57.  The  Duration  of  the  Transit  determined. — The  best 
method  of  finding  the  angular  distance  between  the  two 
paths  across  the  sun  (Fig.  19)  is  to  measure  at  each  sta- 
tion the  exact  time  that  it  takes  Venus  to  cross  the  sun. 
This  is  done  by  noting  down  the  time  when  the  planet 
first  touches  the  edge  of  the  sun,  the  fast  external  con- 
tact (A  in  Fig.  19),  and  also  just  when  it  breaks  away 
from  the  inside  edge 
of  the  sun,  the  fast 
internal  contact  (B). 
From  these  the  time 
when  the  centre  of 
the  planet  crosses  the 
edge  can  be  found. 
This  is  the  beginning 

Of  the  transit.     In  the         FIG.  19.— THE  PATH  OP  VENUS  ACROSS  THE  Stm. 

same  way,  from  the 

second  internal  and  external  contacts,  the  time  when 
the  transit  ends  is  found.  And  since  we  know  the 
angular  distance  that  Yenus  passes  over  in  an  hour, 
multiplying  this  by  the  number  of  hours  occupied  in 

1  The  simple  conditions  here  supposed  arc  never  realized.  Instead 
of  two  stations,  there  are  many,  and  no  two  of  them  are  actually  at 
the  extremities  of  a  diameter.  The  earth  does  not  stand  still  during  the 
transit,  but  rotates  on  its  axis  and  moves  in  its  path  around  the  sun. 

1 


74  ASTRONOMY. 

the  transit  will  give  the  lengths,  and  therefore  the  po 
sitions,  of  the  two  paths.  From  these  their  distance 
apart  (AB  in  Fig.  19)  is  easily  found. 

In  1874  and  1882  the  paths  were  also  determined  by 
taking  during  the  eclipse  a  number  of  photographs  of 
the  sun  with  Venus  upon  it.  Upon  these  photographs 
themselves  the  paths  were  carefully  measured. 

58.  The  Black  Drop. — At  the  two  transits  of  the  last 
century  the  observers  were  greatly  perplexed  at  finding, 
when  the  moment  of  internal  contact  came  and  the 
planet  should  have   separated   itself  from   the   inner 
edge  of  the  sun,  that  it  did  not  do  so,  but  seemed  to 
be  attached  to  the  edge  of  the  sun  for  several  seconds 
by  a  sort  of  neck.      This   ligament   apparently  con- 
necting the  planet  with  the  edge  of  the  sun  is  called 
the  black  drop.     This  made  it  very  difficult  to  determine 
the  exact  time  of  internal  contact.     The  black  drop  has 
been  found  to  have  been  due  mainly  to  the  unsteadiness 
of  the  atmosphere  and  the  imperfections  of  the  tele- 
scopes then  used. 

59.  The  Early  Transits.— The  first  transit  of  Venus* 
ever  observed  occurred  in  1639.     Jeremiah  Horrox,  a 
clergyman  of  the  Church  of  England,  and  only  eighteen 
years  of  age,  calculated  from  the  motions  of  the  earth 
and  Venus  that  there  would  be  a  transit  of  the  planet 
on  a  certain  Sunday  in  November.     He  arranged  his 
telescope  so  as  to  throw  the  sun's  image  upon  a  screen, 
as  explained  in  the  directions  given  for  observing  sun- 
spots  (Art.  41).     During  the  middle  of  the  day  he  had 
to  be  at  church,  but,  when  he  came  back  in  the  after- 
noon, to  his  great  joy  he  found  the  image  of  the  planet 
upon  the  screen.     The  next  transit  occurred  in  1761. 
Astronomers  now  knew  the  importance  of  the  event, 


THE  INFERIOR  PLANETS.  75 

and  preparations  were  made  to  observe  it  in  various 
parts  of  the  world ;  but  the  observations  were  not  satis- 
factory. The  next  one  came  eight  years  later,  in  1769. 
Astronomers  were  scattered  over  all  those  parts  of  the 
world  from  which  it  could  be  seen.  The  observations 
were  thought  to  be  satisfactory,  and  gave  a  distance  of 
95,000,000  of  miles  from  the  sun  to  the  earth.  This 
was  universally  accepted  for  many  years,  and  may  still 
be  found  in  older  text-books. 

In  connection  with  this  transit  occurred  an  incident 
which  well  illustrates  the  devotion  of  scientific  men  to 
their  work.  A  French  astronomer,  Le  Gentil,  had 
been  sent  out  to  India  eight  years  before  to  observe 
the  previous  transit  there.  Owing  to  the  war  between 
France  and  England,  he  was  not  allowed  to  land  in 
British  India.  He  saw  the  transit  on  shipboard,  but 
the  unsteadiness  of  the  ship  prevented  him  from 
making  any  valuable  observations.  As  he  was  there, 
he  determined  to  wait  eight  years  and  observe  the  next 
one.  He  supported  himself  by  business  during  these 
years,  and  made  many  scientific  observations  as  well. 
"  The  long-looked-for  morning  of  June  4,  1769,  found 
him  thoroughly  prepared  to  make  the  observations  for 
which  he  had  waited  eight  long  years.  The  sun  shone 
out  in  a  cloudless  sky,  as  it  had  shone  for  a  number  of 
days  previously.  But  just  as  it  was  time  for  the  tran- 
sit to  begin,  a  sudden  storm  arose,  and  the  sky  became 
covered  with  clouds.  When  they  cleared  away,  the 
transit  was  over.  It  was  two  weeks  before  the  ill-fated 
astronomer  could  hold  the  pen  which  was  to  tell  his 
friends  in  Paris  the  story  of  his  disappointment."1 

1  Newcomb's  Popular  Astronomy. 


76  ASTRONOMY. 


Another  transit  would  not  occur  for  over  a  hundred 
years. 

A  part  of  this  transit  was  visible  in  the  Eastern 
United  States,  and,  under  the  management  of  Ritten- 
house,1  was  observed  at  Norristown,  Pa.,  Philadelphia, 
and  Cape  Hen! open.  Although  neglected  at  the  time 
by  the  European  astronomers,  these  observations  were 
the  most  accurate  that  were  made. 

60.  The  Transit  of  1874.— Before  the  next  transit 
came,  in  1874,  astronomers  were  certain,  from  other 
methods  of  measurement,  that  a  mistake   had   been 
made  in  1769,  and  that  the  sun  was  not  so  far  off  as 
95,000,000  of  miles.      This   transit   was   expected   to 
settle  the  matter,  and  very  extensive  preparations  were 
made  to  observe  it.     The  transit  was  to  occur  while  it 
was  night  in  the,  United  States  and  over  great  part 
of  Europe,  so  Asia  and  the  South  Pacific  Ocean  were 
the  best  places   for  seeing  it.     All  of  the   foremost 
nations  of  the  world  sent  out  expeditions  to  observe  it, 
at  an  expense  altogether  of  $1,000,000.     Our  own  gov- 
ernment gave  $150,000,  and  sent  out  eight  different 
parties  of  observers. 

61.  The  Transit  of  1882.— The  last  transit  of  Venus 
was  observed  on  December  6,  1882.      It  was  visible 
over  nearly  the  whole  American  continent,  and  the  first 
of  it  over  the  eastern  part  of  the  Eastern  continent  as 
well.     In  the  transit  of  1874,  the  European  observers 
had  had  such  poor  results  from  photographing  the  sun 
with  Yenus  on  it  that  they  determined  not  to  try  this 


1  David  Rittenhouse,  1732-1796.  A  farmer's  boy  who  made  himself 
a  great  mathematician  and  astronomer.  He  used  to  calculate  eclipses 
on  his  plough-handles.  His  life  is  interesting  and  instructive. 


THE  INFERIOR  PLANETS.  77 


method  again.  The  Americans,  however,  using  a  dif- 
ferent process  of  obtaining  their  photographs,  were 
more  successful,  and,  though  the  results  of  their  efforts 
in  1874  were  not  all  that  was  expected,  it  is  believed 
that  the  most  valuable  outcome  of  all  the  observations 
will  be  found  in  the  American  photographs. 

These  considerations  induced  them  to  try  the  same 
method  in  1882.  A  series  of  photographs  was  taken 
during  the  transit,  about  two  hundred  at  each  station. 
These  photographs  are  about  four  inches  in  diameter, 
and  show  the  sun  with  Venus  on  its  face  as  a  round 
black  spot.  They  are  taken  to  Washington  and  meas- 
ured at  leisure. 

Eight  parties  were  sent  out  by  the  United  States 
government,  four  to  the  northern  and  four  to  the  south- 
ern hemisphere.  The  day  was,  in  general,  favorable, 
and  good  success  attended  the  efforts  of  the  observers. 
Everything  possible  to  be  gained  by  a  transit  of  Venus 
will  probably  result  from  this  one,  and  it  is  no  great 
cause  of  regret  that  another  will  not  occur  until  the 
year  2004. 

The  results  of  these  two  transits  are  somewhat  dis- 
cordant; and  astronomers  are  coming  to  the  conclu- 
sion that  even  with  the  best  of  instruments,  in  the  hands 
of  the  most  experienced  observers,  observations  of  the 
transit  of  Venus  do  not  give  us  the  best  methods  of 
finding  the  distance  to  the  sun,  but  that  a  method  to 
be  explained  farther  on  is  more  reliable. 

62.  How  to  observeVenus. — When  at  her  best,  Venus 
is  the  brightest  of  all  the  planets,  and  much  brighter 
than  any  of  the  fixed  stars.  She  is  never  in  the 

7* 


78  ASTRONOMY. 


part  of  the  sky  opposite  to  the  sun,  and  at  night  is  only 
to  be  seen  for  a  few  hours  after  sunset  or  before  sun- 
rise. For  about  nine  and  one-half  months  she  is  seen 
after  sunset,  and  is  an  evening  star;  for  the  next  nine 
and  one-half  months  she  is  seen  before  sunrise,  and  is 
a  morning  star.  When  she  is  to  be  seen  in  the  evening, 
and  when  in  the  morning,  are  given  in  all  almanacs. 
These  facts  will  generally  make  it  easy  to  recognize 
Venus.  She  is  not  brightest  at  greatest  elongation,  but 
when  a  little  nearer  the  earth  than  that.  At  this  time 
Venus  casts  a  shadow  on  a  moonless  night,  and  can 
be  seen  in  the  daytime  with  the  naked  eye,  if  one 
knows  just  where  in  the  sky  to  look  for  her.  The 
smallest  telescope  will  show  the  phases  of  Venus,  which 
are  her  most  interesting  features.  Venus's  rapid  mo- 
tions among  the  stars,  both  direct,  that  is,  towards  the 
east,  and  retrograde,  towards  the  west,  should  be  noted. 
(Art.  24). 


THE  EARTH.  79 


CHAPTEK  IV. 

TEE  EARTH,      0 

Distance  from  the  Sun,  93,000,000  Miles.  Average  Diameter, 
7918  Miles.  Length  of  Year,  365^  Days.  Length  of  Day, 
24  Hours.  Specific  Gravity,  5£.  One  Satellite. 

63.  The  Earth's  Shape.— The  earth  is  the  planet 
which  comes  next  to  Venus,  and,  like  the  rest  of  the 
planets,  is  round,  or,  more  properly,  spherical.  The 
facts  which  led  the  ancients  to  believe  that  the  earth 
was  round  have  already  been  given  .(Art.  7),  but  other 
proofs  of  this  fact  are  now  known.  One  of  the  best 
known  of  these  is  that  men  have  frequently  travelled 
around  it  in  almost  every  possible  direction.  This 
proves  that  it  is  rounded,  but  not  that  it  is  certainly  a 
sphere.1 

Another  proof  commonly  given  is  that,  when  we 
watch  a  ship  sailing  away  from  the  land,  we  notice 
that  its  hull  is  hidden  first,  then  its  lower  sails,  and  last 
of  all  its  highest  sails.  The  water  takes  just  the  shape 
of  the  surface  of  the  earth ;  and,  since  this  gradual  dis- 
appearance is  noticed  upon  the  water  everywhere,  and 
ships  disappear  just  as  fast  in  one  direction  as  in  an? 
other,  the  surface  of  the  water  at  least  must  be  round.5* 

1  Why  not? 

a  The  difference  of  level  on  the  surface  of  still  water  is  8  inches  in 
1  mile.  In  2  miles  it  is  not  twice  8  inches,  but  the  square  of  two 
multiplied  by  8  inches,  4  X  8,  or  32  inches.  In  3  miles  it  is  9  X  8, 
or  72  inches,  and  so  on.  If  your  eye  were  at  the  surface  of  the  water, 


80  ASTRONOMY. 


A  still  stronger  proof  is  given  by  the  eclipses  of 
the  moon.  As  will  be  explained  hereafter,  the  moon 
is  eclipsed  by  getting  into  the  earth's  shadow.  "When 
the  moon  is  passing  into  the  shadow  or  coming  out  of 
it,  the  edge  of  the  shadow  as  seen  upon  the  moon  is 
always  round.  Many  hundreds  of  eclipses  of  the  moon 
have  been  seen,~and  at  these  different  times  every  side 
of  the  earth  has  been  turned  towards  the  moon.  The 
earth  must  look  round,  then,  from  every  side,  and  must 
be  a  sphere.1  Another  convincing  proof  of  the  spheri- 
cal shape  of  the  earth  will  be  given  when  the  method 
of  finding  the  size  of  the  earth  is  explained  (^rt.  65). 

Notwithstanding  the  numerous  and  unanswerable 
proofs  of  the  roundness  of  the  earth,  there  still  seem 
to  be  a  few  people  who  deny  it.  A  few  years  since, 
Mr.  John  Hampden,  of  England,  wrote  a  book  to 
prove  that  the  earth  is  flat.  He  afterwards  offered  to 
bet  five  hundred  pounds  (twenty-five  hundred  dollars) 
that  he  was  right.  The  bet  was  taken,  and  to  settle 
the  question  a  part  of  an  English  canal,  where  there 
were  two  bridges  six  miles  apart,  was  chosen.  Half- 
way between  the  bridges  a  rod  was  put  up.  When  a 
telescope  was  set  up  at  one  end  of  th£  six  miles  and 
pointed  towards  the  bridge  at  the  other  end,  the  place 
on  the  rod  which  was  just  as  far  above  the  surface  of 
the  water  as  the  bridges,  was  found  to  projact  several 
feet  above  the  line^  of  sight.  The  referee  in  the  case 
decided  that  this  proved  the  rotundity  of  the'  earth :  so 
Mr.  Hampden  lost  his  money. 

how  tall  a  ship's  mast  would  be  hidden  by  10  miles  of  water  ?     How 
far  out  at  sea  could  a  mountain  3  miles  high  be  seen  ?    (154-f-  miles.) 
1  What  different  shapes  might  the  earth  have  and  yet  look  round 
from  some  sides  ? 


THE  EARTH. 


64.  Apparent_D£tiations  from   the  Roundness   of  the 
Earth. — One  can  hardly  believe  at  first  that  the  earth  is 
round,  when  he  thinks  of  the  hills  and  mountains  scat- 
tered so  thickly  over  its  surface.     But,  when  compared 
with  the  great  earth,  these  irregularities  are  insignifi- 
cant.   If  on  the  surface  of  a  globe  one  foot  in  diameter 
an  elevation  were  constructed  proportionate  in  size  to 
the  highest  mountain  on  the  earth,  it  would  be  less 
than  yj-^  of  an  inch  high,  and  could  not  be  seen  at  all 
one  foot  away.     If  the  loftiest  of  the  Himalayas  or  of 
the  Andes  is  so  trifling,  we  can  see  how  insignificant 
the  hills  and  even  the  mountains  about  us  must  be.     If 
an  exact  model  of  the  earth  one  foot  in  diameter  were 
made,  a  foot  away  it  would  seem  to  be  perfectly  smooth 
and  round.     What  appears  to  us  to  be  the  great  rough- 
ness of  its  surface  does  not,  then,  at  all  destroy  the 
roundness  of  the  earth. 

65.  Size  of  tfwJEarth^ow  determined. — Since  the  earth 
is  a  sphere,  its  circumference  is  a  circle,  and,  like  every 
other  circle,  contains  three  hundred  and  sixty  degrees. 
So,  if  we  can  find  the  length  of  one  of  these  degrees, 
multiplying  that  length  by  360  will  give  the  circumfer- 
ence of  the  earth. 

In  1764-65,  Mason  and  Dixon  (who  came  over  from 
England  to  mark  the  boundary  between  Pennsylvania 
and  Maryland,  still  called  Mason  and  Dixon's  line), 
at  a  point  in  the  southeastern  part  of  Pennsylvania, 
carefully  observed  the  height  of  a  certain  star  above 
the  northern  horizon,  then  measured  a  straight  line 
directly  south  until  the  star  at  the  same  time  of 
day  (why  ?)  was  one  degree  nearer  the  horizon  than 
when  they  started  (Art.  20).  Then  they  knew  that 
they  had  measured  just  one  degree  of  the  earth's  cir- 


82  ASTRONOMY. 


cumference.1  Parts  of  the  earth's  circumference  have 
been  measured  with  the  utmost  exactness  many  times 
in  different  parts  of  the  earth.  In  other  places  the 
whole  length  of  the  line  running  north  and  south  has 
not  been  measured  upon  the  ground,  as  Mason  and 
Dixon  measured  theirs,  but  the  corners  of  a  row  of 
triangles  extending  from  one  end  of  the  line  to  the 
other  have  been  marked.  Then,  by  measuring  one 
side  of  the  first  triangle  and  the  angles  of  all  the  tri- 
angles, the  whole  distance  is  calculated  by  trigonom- 
etry. This  is  a  much  more  accurate  way  than  to  try 
to  measure  the  whole  distance,  on  account  of  the  hills 
and  mountains  that  would  interfere  with  direct  meas- 
urement. Very  long  lines  have  been  measured  in 
this  way,  and  the  number  of  degrees  from  one  end  to 
the  other  found  by  observing  the  stars.  The  average 
length  of  one  degree  of  the  earth's  circumference  has 
been  found  to  be  about  69  miles,  and  the  whole  cir- 
cumference, then,  is  a  little  less  than  25,000  miles. 
These  measurements  of^the  size  of  the  earth  also  prove 
that  it  is  a  sphere. 

66.  The  Earth  Flattened  slightly  at  the  Poles.— In  meas- 
uring these  lines,  it  was  found  that  a  degree  of  a  me- 
ridian near  one  of  the  poles  of  the  earth  is  a  little 
longer  than  a  degree  near  the  equator.  This  shows 
that  the  degree  near  the  pole  is  part  of  a  greater  circle 
than  the  degree  near  the  equator,2  or  that  the  earth  is 

1  Students  who  understand  geometry  should  work  out  the  proof  of 
this. 

2  This  may  be  hard  to  see  ;  if  so,  let  the  student  draw  a  circle  flat- 
tened above  and  below.     He  will  see  that  where  the  equator  bulges 
out  there  is  a  sharper  curve  than  at  the  flattened  parts.     If  the  curva- 
ture of  the  upper  or  lower  part  be  carried  on  around  till  it  meets, 


THE  EARTH.  83 


slightly  flattened  at  the  poles.  Thecircumference  of 
the  earth  which  passes  through  the  poles  is  not,  then,  a 
circle,  but  is  nearly  an  ellipse ;  but  it  is  so  nearly  a  circle 
that,  if  it  were  accurately  drawn,  no  one  could  dis- 
tinguish it  from  a  circle.  The  distance  through  the 
earth  from  north  to  south  pole  is  twenty-six  miles 
less  than  the  diameter  from  one  side  of  the  equator  to 
the  other.  This  is  much  greater  than  the  height  of  any 
mountain,  and,  as  we  shall  learn  hereafter,  the  bulging 
out  of  the  earth's  equator  produces  some  important 
astronomical  effects,  but  it  would  make  no  appreciable 
difference  in  the  shape  of  a  globe.  If,  then,  an  exact 
model  of  the  earth  of  a  moderate  size  were  made,  the 
sharpest  eye  could  not  see  but  that  it  was  exactly  round, 
a  perfect  sphere.  From  the  circumference  the  earth's 
diameter  is  easily  found  by  arithmetic.  Its  average 
length  is  7918  miles. 

67.  Weight  and  Density  of  the  Earth. — Many  attempts 
have  been  made  to  determine  the  weight  of  the  earth. 
The  first  was  made  in  this  way.  As  we  all  know,  a 
plumb-line  points  directly  down  to  the  centre  of  the 
earth.  But  if  the  plumb-line  be  held  near  a  mountain, 
the  attraction  of  the  mountain  pulls  it  a  little  to  one 
side.  If  the  mountain  were  not  there,  the  plumb-line 
would  point  to  a  certain  place  among  the  stars ;  but  the 
attraction  of  the  mountain  makes  it  point  to  a  differ- 
ent place.  The  difference  between  these  two  places 
in  the  sky  shows  how  far  the  mountain  pulls  the  plumb- 
line  aside.  The  size  of  the  mountain  is  then  meas- 
ured, and  the  average  weight  of  the  rocks  that  make 


or  completes  a  circle,  this  circle  will  run  outside  of  the  middle  parts 
of  the  flattened  one. 


84  ASTRONOMY. 


up  the  mountain  is  found ;  these  will  give  the  weight 
of  the  mountain.  Knowing  this,  and  how  much  the 
mountain  pulled  the  plumb-line  from  the  earth's  cen- 
tre towards  itself,  the  weight  of  the  earth  is  calcu- 
lated. Although  this  was  the  first  method  employed, 
it  is  not  altogether  reliable,  because  we  cannot  be  cer- 
tain that  the  weight  of  the  mountain  has  been  correctly 
found. 

The  best  method  of  finding  the  mass  of  the  earth  is  to 
measure  the  force  with  which  a  large  lead  ball  attracts  a 
small  body  to  itself.  The  weight  of  the  small  body  shows 
how  much  the  earth  attracts  it,  for  the  weight  of  any  body 
is  caused  wholly  by  the  earth's  attraction.  Then  if  the 
attracting  force  of  the  lead  ball,  the  attracting  force  of 
the  earth,  and  the  weight  of  the  lead  ball  be  all  known, 
the  mass  of  the  earth  may  be  found.1  This  experiment  has 
been  repeated  many  hundreds  ot  times  with  the  greatest 
care,  and  as  the  result  the  mass  of  the  earth  is  represent- 
ed by  a  weight  of  about  6,000,000,000,000,000,000,000 
tons.  This  shows  that  the  specific  gravity  of  the  earth 
is  about  5J ;  that  is,  it  is  five  and  one-half  times  as 
heavy  as  a  globe  of  water  of  the  same  size.  The  rocks 
and  soil  at  the  surface  of  the  earth  are  not  nearly  so 
heavy  as  this,  being  generally  only  two  or  three  times 
as  heavy  as  water.  The  inside  of  the  earth  must,  then, 


1  To  solve  this  the  distances  of  the  body  from  the  centre  of  the  earth 
and  from  the  hall  ought  to  he  known.  Calling  these  D  and  d,  and 
denoting  by  M  and  m  the  masses  of  the  earth  and  ball  respectively 
we  have,  from  natural  philosophy, — 

Attracting  force  of  ball  :  attracting  force  of  earth  : :  !*  i  |L 

In  this  proporcion  everything  is  known  but  M,  which  may  there- 
fore be  easily  found. 


THE  EARTH.  85 


be  very  much  heavier  than  its  surface.     This  is  due  to 
the  condensation  caused  by  gravitation. 

68.  The  Earth's  Rotation  on_its  Axis. — The  ancients^ 
generally  believed  that  the  earth  stood  still,  and  that  the 
sun  and  stars  revolved  about  it  every  day  from  east  to 
west.  But  it  is  now  knownJJaat  these  motions  of  the 
heavenly  bodies  are  pnly  apparent,  and  are  caused  by 
the  rotation  of  the  earth  upon  its  axis,  from  west  to  east 
once  a  day.  One  proof  that  the  earth  turns  thus  upon 
its  axis  is  that  all  of  the  thousand's  "of  stars  do  thus,  seem 
tojrevolve  aboutthe  earth  in  exactly  the  same  time. 
TKe~distances  ofTEe^stars  vary  g^eaffy,  but  all  are~1at 
enormous  distances  from  us,  so  that  it  is  impossible  to 
suppose  that  they  all  revolve  about  such  enormous  cir- 
cles in  so  short  a  time,  and  in  exactly  the  same  time. 
Tt}e  only  possible  explanation  of  their  apparent  motion* 
is  that  the  earth  turns  on  its  axis  in  the  other  direc_- 
tion. 

Again,  if  the  earth  thus  rotates  upon  its  axis,  the  top 
of  a  tower  must  move  through  a  larger  circle  in  the 
same  time,  and  hence  move  f aster ,  than  the  foot  of  the 
tower.  If  a  stone  were  dropped  from  the  top  of  the 
tower  upon  the  eastern1  side,  in  falling  through  the  air 
it  would  still  keep  the  forward  motion  of  the  top  of  the 
tower.  And,  since  the  top  moves  faster  than  the- bot- 
tom, the  stone  while  falling  would  move  faster^ea^t- 
ward  than  the  bottom,  and  would  strike  the  ground  a 
little  way  east  of  the  foot  of  the  -tower.  If  the  tower 
were  at  the  equator,  and  500  feet  high,  the  stone  would 
fall  about  two  inches  from  its  foot.  This  experiment 
has  been  tried  many  times  from  towers,  and  in  the 

1  Why  not  upon  the  western  side  ? 
8 


86  ASTRONOMY. 


shafts  of  deep  mines,  and  from  it  we  have  another  proof 
of  the  rotation  of  the  earth.- 

Until  recently  astronomers  thought  that  the  time  of 
the  rotation  of  the  earth  remained  exactly  the  same 
from  century  to  century,  and  that  there  was  therefore 
no  change  in  the  length  of  the  day.  But  it  is  now 
thought  that  the  ocean  tides,  which  move  around  the 
earth  in  the  other  direction,  by  their  friction  may  be 
gradually  making  the  rotation  of  the  earth  slower,  and 
thus  slowly  lengthening  the  day.  But  as  a  day  is,  at  the 
most,  only  ^  of  a  second  longer  than  it  was  2500  years 
ago,  we  may  consider  the  length  of  the  day  as  practi- 
cally invariable. 

69.  Revolution  of  the  Earth  about  the  Sun. — As  has 
been  said,  the  sun  seems  to  move  around  the  whole 
sky  among  the  stars  once  a  year.     There  is  abundant 
evidence  that  this,  too,  is  only  apparent,  and  that  it 
is  the  earth  that  really  moves  about  the  sun  in  this 
time. 

70.  The  Shape  of  the*EartKs_Orbit.--If  the  apparent 
size  of  the  sun  is  carefully  measured  with  a  telescope 
every  day  in  the  year,  it  will  be  found  to  be  largest 
about  the  1st  of  January,  and  to  grow  smaller  every 
day  until  about  the  1st  of  July,  when  it  will  be  small- 
est ;  then  it  will  daily  grow  larger  until  about  the  1st 
of  January  again.     As  we   cannot  suppose  the  sun's 
size  to  change  in.  this  way,  we  are  forced  to  conclude 
that  the  "sun  seems  largest  at  the  1st  of  January  be- 
cause we  are  then  nearest  to  it,  and  that  as  it  seems  to 

1  The  student  may  have  heard  of  the  man  who  proposed  to  travel 
from  one  place  to  another  by  going  up.  in  a  balloon,  and  after  waiting 
until  the  earth  turned  around  under  him,  to  come  down.  Why 
would  his  plan  not  succeed  ? 


THE  EARTH.  87 


grow  smaller  from  day  to  day,  we  must  be  going  far- 
ther from  it  every  day ;  about  July  1,  when  it  seems 
smallest,  we  must  be  farthest  from  it,  just  as  a  man 
seems  smaller  the  farther  off  he  is.  If  the  earth's  dis- 
tance from  the  sun  varies  in  this  way,  its  path  about 
the  sun  cannot  be  a  circle  with  the  sun  in  the  centre. 
It  is  an  ellipse,  like  the  paths  of  the  other  planets.  The 
measurements  just  referred  to  show  that  at  the  1st  of 
January  the  sun's  diameter  appears  to  be  ^-.longer  than 
at  the  1st  of  July,  so  that  the  earth  must  be  about  -£$ 
nearer  the  sun  on  the  former  than  on  the  latter  day. 
As  the  average  distance  of  the  earth  from  the  sun  is 
93,000,000  miles,  we  are  more^  than  3,000,000  miles 
nearer  the  sun  at  the  1st  of  January -than  at  the  1st 
of  July. 

71.  The  Eccentricity  of  the  Earth's  Orbit—Its  Effect 
upon  Climate. — Since  the  difference  of  greatest  and  least 
distances  is  3^,  one  must  be  ^  greater  and  the  other 
^j-  less  than  the  average  distance.  This  ^  is  the  eccen- 
tricity of  the  earth's  orbit  (Art.  25).  It  may  seem 
strange  that  we  should  be  nearest  the  sun  in  winter 
and  farthest  from  it  in  summer,  but  we  shall  presently 
learn  that  summer  and  vvinter  are  due  to  ot.Vmr  ram  sea 
than  Distance  from  f.hft  qirn.  Although  these  differ- 
ences of  distance  are  sma^l  compared  with  the  whole 
distance,  and  the  sharpest  eye  -could,  not  distinguish 
the  earth's  orbit  from  a  circle  (Art.  25),  yet  they  are 
really  considerable,  and  the  earth  actually  receives  ^ 
more  heat1  on  the  1st  of  January  than  on  the  1st  of 
July.  This  makes  our2  winters  slightly  warmer  and 


1  For  the  method  of  finding  this,  see  foot-note,  p.  70. 

2  This  refers  to  the  northern  hemisphere. 


88  ASTRONOMY. 


our  summers  slightly  cooler  than  they  would  other- 
wise be.  But,  as  will  be  explained  presently,  after 
some  thousands  of  years  this  will  be  reversed,  and  we 
shall  be  farthest  from  the  sun  in  winter,  and  nearest 
to  it  in  summer. 

72.  Precession  of  the  Equinoxes. — The  student  will  re- 
member that  the  sun  in  its  apparent  yearly  revolution 
around  the  sky  along  the  ecliptic  crosses  the  celestial 
equator  in  two  points  called  the  equinoxes  (Art.  30). 
If  the  equator  were  a  visible  line  in  the  sky,  it  would 
seem  to  be  a  great  circle  among  the  stars,  running  en- 
tirely around  the  earth ;  half  of  it  only  could  be  seen 
at  once.  On  March  20  the  sun  would  be  exactly  on 
this  line,  crossing  it  from  south  to  north ;  the  place  of 
crossing  would  be  the  vernal  equinox.  Day  by  day  the 
sun  would  be  seen  moving  to  the  east  among  the  stars 
(if  they  could  be  seen  at  the  same  time  as  the  sun),  and 
also  getting  farther  and  farther  above  the  line  until 
June  21,  when  it  would  be  23J°  above  or  north  of  the 
line.  Then,  as  it  moved  on  in  its  eastward  course, 
it  would  draw  nearer  and  nearer  to  the  line  again, 
crossing  it  from  north  to  south  on  September  21,  the 
autumnal  equinox.  For  the  rest  of  the  year  its  path 
would  be  a  similar  curve  south  of  thes  equator,  coming 
back  to  cross  the  equator  on  March  20  again.  But  this 
time  the  sun  would  cross  ike  equator  a  little  before  it  came  to 
the  place  where  it  crossed  a  year  before.  The  equinox,  or 
place  where  the  sun  crosses  the  equator,  moves  back- 
ward or  westward  every  year.  The  same  would  hap- 
pen at  the  other  equinox  in  September ;  there,  too,  the 
sun  would  cross  the  equator  before  it  got  to  the  cross- 
ing-place of  the  year  before.  This  moving  backward 
of  the  places  where  the  sun  crosses  the  equator  is 


THE  EARTH.  89 


called  the  precession1  of  the  equinoxes.  This  motion  is 
extremely  slow.  It  would  take  the  vernal  equinox 
about  26,000  years  to  move  entirely  around  the  equa- 
tor once  in  this  way.  The  sun  crossing  the  equator 
only  a  very  little  way  farther  back  every  spring  would 
cross  it  about  26,000  tim^s  before  eoming  to  its  first 
crossing-place  again. 

If  the  earth  were  perfectly  round  there  would  be  no 
such  motion  of  the  equinoxes :  the  sun  would  always 
cross  the  equator  at  the  same  places.  But,  as  we  saw 
in  Art.  66,  the  earth  is  not  quite  .a  perfect  sphere^.  It 
is  flattened  at  the  poles,  or,  which  is  the  same  thing, 
there  is  a  bulging  or  protuberance  about  the  equator. 
It  is  the  attraction  of  this  equatorial  protuberance/  by 
the  sun  and  moon  that  causes  the  precession  of  the 
equinoxes. 

73.  Effects  of  the  Precession  of  the  Equinoxes. — As  has 
been  said  in  Art.  31,  the  right  ascension2  of  a  heavenly 
body  is  the  distance  (in  degrees)  from  the  vernal  equi- 
nox eastward,  or  forward,  to  the  body.  And  as  the  ver- 
nal equinox  moves  backward,  the  right  ascensions  of 
the  stars  must  be  growing  greater.  As  the  precession 
itself  is  so  slow,  this  change  is  also  slow,  but  after 
many  years  it  becomes  considerable,  and  it  was  by  no- 
ticing this  increase  in  the  right  ascensions  of  stars  that 
Hipparchus  (Art.  6)  discovered  the  precession  of  the 
equinoxes  more  than  two  thousand  years  ago.  The 
fact  that  the  first  sign  of  the  ecliptic  does  not  now 
coincide  with  the  first  constellation  of  the  zodiac  (Art. 
32)  is  now  explained.  As  there  stated,  when  the  con- 


1  From  Latin  prcecedere,  to  go  before. 

s  "What  corresponds  to  right  ascension  upon  the  earth  ? 


90  ASTRONOMY. 


stellations  were  named,  the  vernal  equinox  was  prob- 
ably at  the  beginning  of  the  first  constellation,  so  that 
the  signs  of  the  ecliptic  received  the  names  of  the  con- 
stellation in  which  they  lay.  But  since  that  time  the 
equinox  has  moved  nearly  30°  backwards,  and  the  first 
sign  of  the  ecliptic  coincides  with  the  twelfth  constel- 
lation. 

Another  effect  of  the  precession  of  the  equinoxes  is 
to  change  slowly  the  direction  in  which  the  axis  of  the 
earth  points.  The  backward  motion  of  the  equinoxes 
causes  the  north  pole  to  move  around  in  a  circle  once 
in  26,000  years.1  JS~ow  the  north  end  of  the  earth's 
axis  points  almost  directly  to  what  is  called  the  north 
star,  and  it  will  continue  to  point  almost  towards  it 
for  many  years  to  come.  But  the  earth's  axis  has  not 
always  pointed  towards  this  star,  nor  will  it  always 

1  This  motion  of  the  earth's  axis  and  the  whole  subject  of  the  preces- 
sion of  the  equinoxes  constitute  one  of  the  most  difficult  points  in 
astronomy.  The  following  illustration  may  help  to  make  it  clear : 

Take  an  apple  to  represent  the  earth.  Call  the  stem  the  north  pole, 
and  mark  a  line  around  the  middle  of  the  apple  for  the  equator.  If 
this  apple  be  floated  in  a  bucket  of  water  so  that  just  half  of  it  is 
above  the  water,  with  the  stem  leaning  about  23J°  from  the  per- 
pendicular, the  position  of  the  earth  is  well  represented.  The  sur- 
face of  the  water  is  the  ecliptic,  and  where  the  apple's  equator  crosses 
the  water-line  are  the  equinoxes.  If  the  apple  be  now  twisted  around 
so  that  the  stem  shall  move  in  a  circle,  leaning  in  every  direction,  but 
always  about  23^°  from  the  perpendicular,  half  of  the  apple  being 
always  in  the  water,  this  revolution  of  the  north  pole  is  represented. 
And  it  will  be  seen  that  the  surface  of  the  water  is  continually  cross- 
ing the  apple's  equator  in  new  places  as  the  apple  turns.  This  repre- 
sents the  precession  of  the  equinoxes. 

This  motion  has  also  been  compared  to  the  motion  of  a  top  when 
it  is  "  dying  out."  It  then  leans  outward  and  slowly  revolves.  One 
such  revolution  of  the  top  represents  the  26,000  year  revolution  of 
the  earth's  axis. 


THE  EARTH. 


91 


do  so  in  the  future.  And,  as  it  moves  slowly  around 
in  its  journey  of  26,000  years,  it  will  point  in  turn 
to  every  star  that  lies  in  its  circular  path.  So  that 
future  generations  will  have  to  use  other  stars  for 


Mar* 


FIG.  20.— THE  26,000  YEAR  PATH  OF  THE  NOBTH  POLE 
AMONG  THE  STARS. 

their  north  stars.  Fig.  20  shows  the  path  of  the  north 
pole  among  the  stars  as  caused  hy  the  precession  of 
the  equinoxes.  Some  of  the  stars  which  generations 
in  the  far  future  will  prohahly  use  as  north  stars,  are 
marked. 

74.  Nutation. — If  the  attraction  of  the  sun  and  moon 
upon  the  protuberance  about  the  earth's  equator  were 
always  the  same,  the  precession  of  the  equinoxes  would 
cause  the  north  pole  to  revolve  in  a  perfect  circle. 
But,  on  account  of  its  own  motions,  the  moon's  at- 
traction is  always  changing,  growing  greater  and  less 
alternately.  This  causes  the  pole  to  revolve  in  a  wavy 
curve,  and  not  in  a  perfect  circle,  as  represented  in 


92  ASTRONOMY. 


Fig.  20.  The  real  path  of  the  north  pole  in  the  sky  is 
a  circular,  wavy  line,  crossing  and  recrossing  the  circle 
in  Fig.  20.  But  these  wavings  are  so  small  that  they 
cannot  be  shown  in  the  circle  in  Fig.  20.  Yet  they  are 
of  much  importance  in  astronomy.  It  is  this  waving 
backward  and  forward  that  is  called  nutation.1 

75.  The  Seasons.— When  the  sun  is  nearly  overhead 
it  gives  us  much  more  heat  than  when  it  is  far  down 
in  the  sky.  This  is  proved  every  day.  In  the  morn- 
ing or  in  the  evening  the  sun's  rays  are  very  slanting, 
and  it  is  much  cooler  than  at  notfn,  when  the  sun  shines 
much  more  directly  down  upon  us. 

Fig.  21  represents  two  rays  of  equal  breadth  striking 

the  earth  at  noonday  and 

10  RAY  AY  at   evening  respectively. 

It  is  plain   that  the  in- 
clined or  evening  ray  has 
to  warm  a  much  larger 
surface  (BC)  than  the  di- 
rect or  noonday  ray;  and 
each  spot  cannot,  there^ 
fore,    receive    so    much 
heat. 
, ^  The  changing  seasons 

PIG.  21.— SUN'S  RAYS  STRIKING  THE  EARTH    are  due  to  the  Same  CaUS6, 
AT  DIFFERENT  INCLINATIONS.  ,,  .  ,. 

the  varying  inclinations 

of  the  sun's  rays.  Fig.  22  represents  the  earth  with 
its  important  circles  drawn  upon  it.  It  will  help  to 
explain  the  seasons.  On  March  20  the  sun  is  directly 
over  the  equator,  exactly  in  front  of  the  middle  of  the 
figure.  And  since  the  sun  lights  up  just  half  of  the 

1  Nu-ta'tion,  from  Latin  nutatio,  nodding. 


THE  EARTH. 


93 


earth  at  one  time,  its  light  extends  both  to  the  north 
and  south  poles.  If  the  figure  be  held  up  just  in  front 
of  the  eyes,  and  be  supposed  to  rotate  about  the  axis 
from  left  to  right,  or,  much  better,  if  a  globe  be  held  in 


FIG.  22.— THE  EARTH  AND  ITS  IMPORTANT  CIRCLES. 

this  position  and  made  to  rotate,  it  will  be  seen  that, 
since  the  sun  is  on  the  equator,  it  will  rise  directly  east 
of  us  at  six  o'clock  in  the  morning,  and  set  directly 
west  of  us  at  six  o'clock  in  the  evening.  Day  and 
night  will  be  equal  all  over  the  earth.  (See  Fig.  23.) 
Hence  the  name  of  this  time  of  year,  the  equinox.  This 
is  the  beginning  of  spring,  so  it  is  the  vernal  equinox. 
As  the  sun  moves  on  in  its  yearly  journey  around  the 


94  ASTRONOMY. 


sky,1  it  is  of  course  always  directly  over  the  line  marked 
ECLIPTIC  in  the  figure. 

Moving  from  left  to  right,  the  sun  in  three  months 
goes  one-fourth  way  around  its  circular  path,  and  on 


FIG.  23.— DAT  AND  NIGHT  AT  THE  EQUINOXES. 

June  21  is  to  the  right  of  the  figure  (22),  directly  beyond 
D.  As  the  sun  is  now  23J°  above  the  equator,  it  shines 
around  23J°  beyond  the  north  pole  to  A ;  but  below 
the  equator  it  shines  only  to  B,  23J°  short  of  the  south 
pole.  Right  of  AB  is  day,  left  of  AB  is  night.  As 

1  It  must  not  be  forgotten  that  this  apparent  motion  of  the  sun 
about  the  earth  is  really  due  to  the  earth's  yearly  motion  about  the 
sun,  and  that  this  causes  the  change  of  seasons.  But  the  explanation 
is  simpler  if  the  sun  be  supposed  to  revolve  about  the  earth.  In  the 
same  way  we  commonly  say,  "  The  sun  rises,"  or  "The  sun  sets," 
although  these  things  are  really  due  to  the  rotation  of  the  earth. 


THE  EARTH.  96 


the  earth  rotates  while  the  sun  is  here,  the  sun  rises  in 
the  northeast  and  sets  in  the  northwest,  and  we  in  the 
north  temperate  zone  have  our  longest  days  and  short- 


FIG.  24. — DAT  AND  NIGHT  AT  THE  SUMMER  SOLSTICK. 

est  nights.  This  is  the  beginning  of  summer, — the 
summer  solstice.  (See  Fig.  24.) 

For  the  next  three  months  the  sun  moves  on  around 
the  earth,  now  of  course  back  of  the  figure  (22),  and  on 
September  22  is  directly  behind  the  middle  of  the  fig- 
ure and  on  the  equator,  making  day  and  night  again 
equal  everywhere.  This  is  the  beginning  of  autumn, 
— the  autumnal  equinox.  (See  Fig.  23.) 

In  three  months  more  the  sun  is  directly  to  the  left 
of  C,  23J°  south  of  the  equator.  Now  the  half  of  the 
earth  to  the  left  of  AB  is  lighted  up,  while  the  half  to 
the  right  is  in  darkness.  The  sun  shines  23J°  beyond 
the  south  pole,  and  not  at  all  within  23|°  of  the  north 


96  ASTRONOMY. 


pole.  Now,  as  the  earth  rotates,  the  sun  rises  in  the 
southeast  and  sets  in  the  southwest;  our  days  are 
shortest  and  our  nights  longest.  It  is  the  winter  solstice, 
— the  beginning  of  winter.  Another  three  months,  and 
the  sun  comes  to  the  vernal  equinox  again. 

76.  The  Seasons  in  the  Southern  Hemisphere. — South 
of  the  equator  the  seasons  are  exactly  opposite  to  ours. 
When  the  sun  is  nearest  over  our  heads,  and  shines 
most  directly  down  upon  us,  in  the   southern  hemi- 
sphere it  is  lowest  down  in  the  sky,  and  its  rays  are 
most  slanting.     There  the  winter  months  are  June, 
July,  and  August;    the  summer  months,  December, 
January,  and  February. 

77.  Why  the  Days  and  Nights  are  Unequal. — Figs.  22 
and  24  show  wEy^  except  at  the  equator,  day  and  night 
are  generally  unequal.     When  the  sun  is  to  the  right 
of  D  (Fig.  22),  it  shines  upon  more  than  half  of  each  of 
the  northern  parallels  of  latitude  (the  tropic  of  Cancer, 
for  instance) ;  and  therefore  every  place  north  of  the 
equator,  since  it  revolves  every  day  through  a  circle  of 
latitude,  has  sunlight  for  more  than  half  of  the  twenty- 
four  hours.     It  is  evident,  too,  that  the  farther  north 
one  goes,  the  greater  is  the  part  of  the  circle  in  the  sun- 
light, and  therefore  in  summer  the  days  grow  longer 
and  the  nights  shorter  as  we  go  north.1     At  the  pole 
itself  the  sun  shines  for  half  the  year,  for  from  the 
vernal  to  the  autumnal  equinox  the  sun  is  north  of  the 
equator,  and  therefore  less  than  90°  from  the   north 
pole,  so  that  it  must  always  shine  upon  the  pole.     At 
other  places  in  the  frigid   zone   the  sun  shines  day 

1  In  Edinburgh,  Scotland,  on  June  21,  the  sun  rises  about  half-past 
three  in  the  morning,  and  does  not  set  until  half-past  eight  in  the 
evening,  while  twilight  lasts  all  night. 


THE  EARTH.  97 


after  day  without  setting,  the  time  being  greater  as  the 
place  is  nearer  to  the  pole.  South  of  the  equator  the 
nights  are  longer  than  the  days,  and  the  south  pole  is 
having  a  six-months'  night.  Six  months  later  the  con- 
ditions are  reversed.  The  people  of  the  southern  hemi- 
sphere have  the  long  days  and  we  the  long  nights. 

78.  Duration  of  the  Seasons. — Astronomically,  the  sea- 
sons begin  and  end  with  the  equinoxes  and  solstices. 
Spring  begins  March  20,  and  ends  June  21  ;J  summer 
begins  June  21,  and  ends  September  22;  and  so  on 
through  the  year.     Our  almanacs  agree  with  this  divi- 
sion of  the  year ;  but  popularly,  in  the  United  States, 
spring  begins  March  1,  and  summer  June  I.2 

If  the  number  of  days  from  the  vernal  to  the  au- 
tumnal equinox  be  counted,  it  is  found  to  be  seven 
more  than  the  number  from  the  autumnal  to  the  vernal 
equinox.  And  if  the  time  be  counted  more  exactly,  it 
is  found  that  the  sun  is  north  of  the  equator  about  eight 
days  longer  than  he  is  south  of  it.  This  is  due  to  the 
fact  that  the  sun  is  nearer  to  one  end  of  the  earth's 
orbit  than  to  the  other ;  and  the  earth  moves  through 
the  larger  end  of  its  orbit  in  our  spring  and  summer, 
taking  a  longer  time  to  do  it.  In  the  southern  hemi- 
sphere this  is  reversed.  There  spring  and  summer  are 
eight  days  shorter  than  autumn  and  winter.  This 
seems  to  make  the  temperature  of  the  northern  hemi- 
sphere milder  than  that  of  the  southern.  In  time  the 
precession  of  the  equinoxes  will  reverse  these  conditions. 

79.  Causes  of  Heat  and  Cold  at  Various  Seasons. — As 

1  Remember  that  these  may  vary  a  day  (see  note  on  p.  40). 

8  In  England,  February,  March,  and  April  are  commonly  called 
the  spring  months,  May,  June,  and  July  the  summer  months,  and  so 
on  through  the  year. 

0 


98  ASTRONOMY. 


has  been  shown,  the  sun  shines  most  directly  down 
upon  us  in  summer,  and  most  obliquely  in  winter.  This 
would  make  our  summers  warmer  than  our  winters. 
Besides,  when  the  sun  shines  most  obliquely  upon  us, 
the  rays  of  heat  pass  through  a  greater  thickness  of 
air,  which  absorbs  more  of  their  heat,  leaving  less  to 
reach  the  earth.1  A  third  reason  is  that  in  summer 
the  days  are  longer  than  in  winter. 

80.  Why  the  Greatest  Heat  and  Cold  occur  after  the  Sol- 
stices.— On  June  21  the  sun  shines  most  directly  down 
upon  us,  and  also  for  the  longest  time ;  it  may  seem 
strange,  then,  that  we  have  our  hottest  weather  not 
at  that  time,  but  several  weeks  later.     It  is  true  that 
we  are  getting  the  most  heat  from  the  sun  on  June  21, 
but  for  several  weeks  after  that  day  we  receive  more 
heat  than  we  lose  by  radiation,  and  the  weather  grows 
hotter;  just  as  a  man  who  earns  even  a  little  more 
than  he  spends  is  constantly  growing  richer.     About 
June  21  our  savings  of  heat  are  the  largest,  for  we  then 
get  most,  but  we  are  still  saving  some,  though  less  and 
less  every  day,  for  several  weeks,  and  are  thus  growing 
richer  in  heat  until  the  latter  part  of  July  or  the  begin- 
ning of  August.     In  the  same  way  our  coldest  weather 
generally  comes  not  at  the  winter  solstice,  but  perhaps 
a  month  later.     On  December  21  we  receive  the  least 
heat  from  the  sun,  but  for  some  weeks  afterwards  we 
continue  to  lose  more  than  we  get,  and  are  growing 
colder.     For  the  same  reason  the  hottest  part  of  the 
day  is  not  at  noon,  but  some  time  in  the  afternoon. 

81.  Geographical  Zones. — Fig.  22  also  shows  the  well- 
known  geographical  zones.     The  torrid  zone  extends 

1  Construct  a  figure  showing  this. 


THE  EARTH.  99 


23J°  on  each  side  of  the  equator.  Its  boundaries  are 
the  lines  where  the  sun  turns  back  at  the  solstices.  Its 
upper  boundary  is  called  the  tropic  of  Cancer ;  the  sun 
is  directly  over  this  line  on  the  21st  of  June.  It  then 
enters  the  fourth  sign  of  the  ecliptic,  Cancer  (Art.  30), 
from  which  the  tropic  is  named ;  the  sun  now  begins 
to  go  back  towards  the  equator.  Six  months  later  it 
has  gone  entirely  across  the  torrid  zone  to  the  tropic 
of  Capricorn,  which  takes  its  name  from  that  of  the 
tenth  sign  of  the  ecliptic,  which  the  sun  now  enters 
and  begins  its  journey  back  towards  the  equator  again. 
The  torrid  zone,  then,  is  the  part  of  the  earth's  sur- 
face which  the  sun  some  time  in  the  year  shines  directly 
down  upon.  It  is  the  hottest  part  of  the  earth's  sur- 
face. In  Art.  5,  when  the  sun  was  supposed  to  be  to 
the  right  of  D,  in  Fig.  22,  it  was  seen  that  the  sunshine 
would  extend  23J°  beyond  the  north  pole  to  A.  As 
the  earth  rotates,  the  sun  constantly  shines  over  all  the 
area  within  23J°  of  the  north  pole.  The  circle  which 
is  everywhere  23J°  from  the  north  pole  is  the  boundary 
line  between  the  north  frigid  and  the  north  temper- 
ate zones.  It  is  the  Arctic  circle.  At  all  points  of 
this  circle  the  sun  would  just  escape  setting  on  June 
21.  At  the  same  time  the  sun  does  not  shine  within 
23J°  degrees  of  the  south  pole.  This  is  the  south 
frigid  zone,  and  at  the  Antarctic  circle  on  the  21st 
of  June  the  sun  does  not  rise. 

Six  months  later  the  sun  is  at  the  winter  solstice : 
now  the  sun  does  not  rise  in  the  north  frigid  zone, 
and  does  not  set  in  the  south  frigid  zone.  Here, 
when  they  shine  at  all,  the  sun's  rays  are  always  very 
oblique,  and  these  are  the  coldest  portions  of  the  earth's 
surface. 


100  ASTRONOMY. 


The  temperate  zones  lie  between  the  frigid  and 
torrid  zones.  Here  the  sun  is  never  directly  over- 
head, yet  it  rises  and  sets  every  day  during  the  year. 
Their  name  fitly  describes  the  temperature  of  these 
zones. 

82.  Effects  of  a  Change  in  the  Angle  between  the  Equator 
and  the  Ecliptic. — If  the  angle  between  the  equator  and 
the  ecliptic  should  increase  (see  Fig.  22),  the  torrid  and 
frigid  zones  would  widen,  while  the  temperate  zones 
would  grow  narrower.  The  sun  would  rise  higher  in 
the  sky  in  summer,  and  sink  lower  in  winter.  This 
would  make  our  summers  hotter  and  our  winters  colder. 
If  the  angle  should  decrease,  the  torrid  and  frigid 
zones  would  decrease  also,  but  the  temperate  zones 
would  widen.1  The  sun  would  not  rise  so  high  in  the 
sky  in  summer  or  sink  so  low  in  winter.  Our  sum- 
mers would  be  cooler,  our  winters  warmer.  If  there 
were  no  angle  between  the  two,  the  sun  would  always 
be  directly  over  the  equator,  day  and  night  would  be 
equal  everywhere  through  the  whole  year,  and  there 
would  be  no  change  of  seasons.  The  equator  of  the 
planet  Jupiter  makes  a  very  small  angle  with  its  orbit, 
so  that  there  can  be  scarcely  any  change  of  seasons 
upon  it,  while  on  Mars  the  angle  is  somewhat  greater 
than  upon  the  earth,  and  there  the  changes  are  greater 
than  here.  This  angle  between  the  equator  and  the 
ecliptic  is  called  the  Obliquity  of  the  Ecliptic. 

As  a  matter  of  fact,  t'his  angle  does  change  slightly, 
but  the  change  is  very  slow,  and  will  never  make  much 
difference  in  the  size  of  the  angle,  so  that  from  tins  cause 

1  How  many  degrees  wide  woul  1  the  different  zones  l»e  if  the  angle 
between  the  t  quator  and  ihe  ecliptic  were  15°?  30°?  49°?  45°? 


THE  EARTH.  '>*''      JQl 


there  will  never  be  any  considerable  change  in  our 
seasons.1 

83.  Measures  of  Time. — The  three  most  natural  divis- 
ions of  time  are  the  year,  which  is  the  time  the  earth 
takes  to  revolve  about  the  sun ;  the  month,  which  is 
based  upon  the  time  the  moon  takes  to  revolve  about 
the  earth ;  and  the  day,  which  is  the  time  of  the  ro- 
tation of  the  earth  upon  its  axis.     We  see,  thus,  that  all 
our  measures  of  time  depend  upon  astronomy.     The 
finding  and  keeping  of  correct  time  all  over  the  earth 
is  always  done  by  astronomy,  and  is  one  of  its  most 
valuable  uses. 

84.  The  Sidereal  Day. — From  the  time  a  star  crosses 
our  meridian2  until  it  crosses  it  again  is  called  a  side- 
real3  day.     This  is  the  exact  time  in  which  the  earth 
turns  on  its  axis.     As  has  been  said,  this  time  is  prac- 
tically invariable.     It  may  be  well  to  recall  the  fact 
that  the  word  "day"  is  used  in  two  senses.     As  op- 
posed to  night,  it  means  the  period  of  daylight,  about 
twrelve  hours ;  but  as  used  here  and  upon  several  pages 
following  this,  it  includes  both  daylight  and  darkness, 
twenty-four  hours. 

85.  The  Apparent  Solar  Day. — If  the  sun,  like  a  star, 


1  The  angle  between  the  equator  and  the  ecliptic  is  now  decreasing 
about  45X/  every  hundred  years.     This  will  continue  for  many  cen- 
turies ;  then  it  will  grow  greater  again,  and  so  vibrate  backward  and 
forward.     The  angle  will  never  be  as  much  as  one  and  one-half  de- 
grees greater  or  less  than  at  present,  and  will  be  thousands  of  years 
in  making  one  such  vibration. 

2  A  star  is  said  to  be  on  our  meridian  when  it  is  directly  over  the 
meridian  of  the  earth  passing  through  the  place  where  we  are.     If 
the  star's  declination  (Art.  31)  is  equal  to  our  latitude,  it  is  then  ex- 
actly overhead  ;  if  not,  it  is  directly  north  or  south  of  us. 

<  Side'real,  from  Latin  sidus,  a  star. 

9* 


102  ASTItVNOMY. 


were  always  at  the  same  place  in  the  sky,  a  solar  day 
would  be  just  the  same  as  a  sidereal  day.  But  we  have 
learned  that  the  sun  moves  entirely  around  the  heavens 
every  year;  that  is,  it  moves  through  360°  in  365 
days,  or  about  1°  every  day,  towards  the  east;  this 
distance  is  about  twice  the  sun's  diameter,  for  the  sun's 
diameter  is  about  one-half  of  a  degree.  Now,  the  earth 
rotates  on  its  axis  in  the  same  direction,  and  when  its 
rotation  has  brought  us  around  under  the  sun's  place 
of  the  day  before,  the  sun  has  moved  one  degree  farther 
east,  and  the  earth  must  turn  that  much  farther  to 
bring  us  under  the  sun  again.  Thus  the  time  from 
noon  to  noon  by  the  sun,  which  is  the  apparent  solar 
day,  is  about  four  minutes  longer  than  the  sidereal 
day.1 

86.  The  Apparent  Solar  Days  not  Equal  in  Length. — 
It  has  just  been  shown  that  the  solar  day  is  longer  than 
the  sidereal  day,  because  the  earth  has  to  turn  a  little 
farther  than  a  complete  rotation  to  catch  the  sun.     But 
these  forward  movements  of  the  sun  are  not  the  same 
every  day,  so  that  the  earth  does  not  turn  the  same  dis- 
tance every  day  to  catch  the  sun,  and  the  solar  days  are 
therefore  unequal.    There  are  two  reasons  why  the  sun 
does  not  move  the  same  distance  forward  every  day. 

87.  First  Cause  of  Inequality  in  Solar  Days. — As  shown 
in  Art.  29,  when  the  earth  is  in  perihelion  (Art.  25)  it 


1  As  Proctor  points  out,  this  fact  bears  upon  a  curious  error  often 
found  in  our  geographies  and  other  text-books.  It  is  commonly  said 
that  while  the  earth  revolves  about  the  sun  once  it  rotates  upon  its 
axis  365£  times.  In  fact,  the  earth  rotates  366J  times  during  the 
year,  although  there  are  but  365 J  days  in  the  year.  For  the  time  of 
rotation  is  four  minutes  less  than  a  day,  as  explained  above,  and  there- 
fore there  is  one  more  rotation  than  there  are  days. 


THE  EARTH.  103 


moves  more  rapidly  than  in  any  other  part  of  its  orbit, 
while  at  aphelion  it  moves  most  slowly.  Since  the  sun's 
apparent  motion  among  the  stars  is  really  the  earth's 
motion,  about  January  1,  when  the  earth  is  at  peri- 
helion, the  sun  will  move  farther  every  day  than  usual. 
This  will  be  further  increased  by  the  fact  that  because 
the  sun  is  then  nearest  to  us,  it  will  seem  to  move  still 
more  rapidly.  About  the  1st  of  January,  then,  the 
solar  days  are  longer  than  the  average.  And  since 
about  the  1st  of  July  the  sun  really  moves  more  slowly 
than  usual,  and  from  its  great  distance  seems  to  move 
still  more  slowly,  the  days  then  are  shorter  than  usual. 

88.  Second  Cause  of  Inequality  in  Solar  Days. — The 
ecliptic  in  which  the  sun  moves  is  inclined  to  the  equa- 
tor, and  when  the  sun  is  near  the  equinoxes  its  motion 
of  a  degree  a  day  is  on  the  hypothenuse  of  a  right-angled 
triangle,  but  our  eastern  motion  to  overtake  him  is 
parallel  to  the  equator,  along  the  base  of  the  triangle, 
and  not  so  great.     This  would  make  the  days  about 
the  equinoxes  shorter  than  the  average. 

At  the  solstices  the  sun  moves  nearly  in  the  tropics 
of  Cancer  and  Capricorn,  parallel  to  the  equator.  And 
as  the  sun  moves  through  his  regular  daily  distance 
along  these  small  circles,  it  moves  more  than  a  degree 
along  them  each  day.  This  makes  the  days  about  the 
solstices  longer  than  the  average. 

89.  Mean  Solar  Day. — On  account  of  these  constant 
changes  in  the  length  of  the  apparent  solar  day,  it  is 
not  a  good  measure  of  time.    But  the  average  or  mean 
of  all  the  solar  days  in  the  year  is  taken  as  the  standard 
day.     This  is  the  day  which  our  clocks  and  watches 
keep,  and  which  is  divided  into  twenty-four  hours,  and 
these  into  minutes  and  seconds. 


104  ASTRONOMY. 


90.  The  Equation  of  Time. — Sun  time  is  got  from  the 
sun  either  by  a  sun-dial,  or  by  setting  the  clock  at  12 
when  the  sun  is  exactly  on  the  meridian.  From  the 
two  causes  given  in  Arts.  87  and  88,  sun  time  agrees 
with  true  or  mean  time  only  on  four  days  of  the  year. 
These  are 

APRIL  15,  JUNE  15,  SEPTEMBER  1,  DECEMBER  24.1 

On  the  following  intervening  days,  the  difference 
between  true  time  and  sun  time  is  greatest : 

FEBRUARY  10,  sun  time  15  minutes  slow. 
MAY  14,        "          4        "         fast. 

JULY  25,        "          6       "         slow. 

NOVEMBER    2,         "        16       "         fast.1 

The  difference  between  true  time  and  sun  time  Is 
called  the  equation  of  time.  Our  common  almanacs  give 
the  equation  of  time  for  every  day  of  the  year  in  a  col- 
umn on  the  page  which  gives  the  calendar  of  the  month. 
In  getting  the  time  from  a  noon-mark  or  sun-dial,  the 
time  as  thus  found  must  be  corrected  as  indicated  in 
the  almanac.2  The  times  of  sunset  and  sunrise  as  given 
in  the  almanac  are  in  mean  or  true  time  at  the  lati- 
tude for  which  the  almanac  is  calculated.3  This  time 

1  These  dates  may  vary  slightly. 

3  If  the  column  in  the  almanac  is  headed  SUN  SLOW,  the  number 
of  minutes  must  be  added  to  sun  time.  If  it  is  headed  SUN  FAST, 
they  must  be  subtracted. 

3  According  to  the  almanac,  forenoon  and  afternoon  are  seldom  of 
the  same  length.  The  time  from  sunrise  until  apparent  noon  (when 
the  sun  is  on  the  meridian)  is  just  the  same  as  the  time  from  apparent 
noon  until  sunset.  But  mean  noon  is  understood  in  the  almanac.  If 
the  sun  is  slow,  he  rises  too  late,  and  the  forenoon  is  shorter  than  the 
afternoon.  If  fast,  the  sun  rises  early,  and  the  forenoon  is  the  longer. 


THE  EARTH.  105 


will  be  exact  only  where  the  sun  rises  over  a  level  sur- 
face. 

91.  How   Time  is  Found. — Time  is   sometimes  got 
from  the  sun  as  just  mentioned,  but  it  is  generally  and 
most  accurately  obtained  by  observations  of  the  stars  at 
astronomical  observatories.     By  using  a  small  telescope 
called  a  Transit  (Art.  249),  we  can  find  out  just  when 
some  well-known  star  crosses  the  meridian.     The  time 
when  this  ought  to  occur  is  given  in  the  Nautical  Alma- 
nac (seep.  170),  and  if  the  clock  does  not  show  the  same 
time,  we  know  how  much  too  fast  or  too  slow  it  is. 

In  1883,  by  agreement  of  the  railroad  managers,  four 
time-centres  were  established  in  the  United  States,  viz., 
the  meridians  5  hrs.  (75  degrees),  6  hrs.,  7  hrs.,  and  8 
hrs.,  west  of  Greenwich.  All  our  railroads,  and  our 
time-pieces  generally,  now  run  upon  the  time  of  the 
nearest  or  most  convenient  of  these  meridians.  Correct 
time  is  sent  along  the  railroads  every  day  by  telegraph  ; 
and  in  many  cities  large  balls,  called  time-balls,  are  let 
fall  from  high  points  exactly  at  noon  each  day. 

92.  Civil  and  Astronomical  Days. — Civil  or  ordinary 
days  begin  and  end  at  midnight,  and  are  divided  into 
two  equal  parts,  each  twelve  hours  long.     The  hours 
from  midnight  to  noon  are  marked  A.M.,  the  first  letters 
of  the  Latin  words  ante  meridiem, "  before  noon."    Thus, 
5  A.M.  means  five  o'clock  in  the  morning.     The  hours 
from  noon  to  midnight  are  marked  P.M.,  the  first  letters 
of  the  Latin  words  post  meridiem,  "after  noon." 

The  day  used  in  astronomical  work  begins  and  ends 
at  noon,1  twelve  hours  later  than  the  beginning  and 

1  Why  is  it  most  convenient  for  the  civil  day  to  begin  at  mid- 
night ?  Why  is  it  most  convenient  for  the  astronomical  day  to  begin 
at  noon  ? 


106  ASTRONOMY. 

ending  of  the  same  civil  day.1  This  day  is  usually 
divided  into  twenty-four  hours,  which  are  numbered 
from  1  to  24.  For  many  purposes  an  astronomer  uses 
the  sidereal  day,  which  is  about  four  minutes  shorter 
than  the  mean  solar  day. 

93.  The  Week. — This  is  not  a  natural  astronomical 
division  of  time,  although  a  very  ancient  one.     The 
names  of  the  seven  days  of  the  week  were  derived  as 
follows :  Sunday  is  the  sun's  day ;  Monday,  the  moon's 
day;    Tuesday,   Wednesday,   Thursday,   Friday,   and 
Saturday  are  derived  from  the  names  of  five  old-Eng- 
lish deities. 

94.  The  Month. — The  month  is  a  very  ancient  di- 
vision of  time.     At  first  it  lasted  from  one  new  moon 
until  the  next,  but  this  is  about  twenty-nine  and  one- 
half  days,  a  number*  inconvenient  in  itself  and  not  an 
exact  divisor  of  the  year.     Presently  the  year  was  di- 
vided into  twelve  months  differing  somewhat  in  length. 
The  present  arrangement  of  the  days  in  each  month 
was  made  by  Augustus,  Emperor  of  Rome  at  the  begin- 
ning of  the  Christian  era.     The  names  of  the  first  six 
months  of  the  year  are  derived  from  Roman  names, 
mostly  deities,  July  and   August  are   named  for  the 
Roman   emperors   Julius    Csesar   and   Augustus,  and 
the  last  four  months  are  named  from  the  Latin  words 
meaning  seven,  eight,  nine,  and  ten,  for  when  these 


1  Thus,  July  24,  9  A.M.,  civil  time,  would  be  July  23  d.  21  h.,  as. 
tronomical  time;  and  July  24,  3  P.M.,  civil  time,  would  be  July  24 
d.  3  h.,  astronomical  time. 

What  astronomical  times  correspond  to  these  civil  times  ?  April 
6,  3  A.M.  ;  May  10,  12  (noon) ;  May  10,  12  (midnight). 

"What  civil  times  correspond  to  these  astronomical  times  ?  July 
6  d.  6  h. ;  September  8  d.  14£  h. ;  March  3  d.  0  h. 


THE  EARTH.  107 


names  were  given  there  were  but  ten  months  in  the 
Roman  year. 

95.  The   Year. — The  year  which  is  always  used  is 
the  time  that  it  takes  the  sun  to  pass  from  the  vernal 
equinox  around  to  the  vernal  equinox  again.     This  is 
365  days,  5  hrs.,  48  min.,  46  sec.1     These  odd  hours 
and  minutes  gave  the  ancients  a  great  deal  of  trouble. 
Many  devices  were  used  by  the  different  nations  of  an- 
tiquity to  make  the  different  seasons  come  at  the  same 
time  year  after  year. 

96.  The  Julian  Calendar. — Julius   Caesar  found  the 
Roman  calendar  very  much  in  error.     Their  winter 
months  came  in  autumn,  and  the  1st  of  September 
came   at  the  summer  solstice.     With  the  aid  of  an 
Egyptian  astronomer  he  made  the  ordinary  year  to 
contain  365  days,  but  he  added  one  more  day  to  every 
fourth  year,  and  also  made  the  year  begin  with  Jan- 
uary 1.     If  the  year  were  exactly  365  days  and  6  hours 
long,  this  arrangement  would  be  perfect.     But  because 
the  odd  hours  and  minutes  are  a  little  less  than  one- 
fourth  of  a  day,  the  Julian  years  are  a  little  too  long, 
and  the  calendar  fell  back  about  3  days  every  400  years.2 

1  This  is  called  the  tropical  year,  to  distinguish  it  from  the  sidereal 
year,  the  time  occupied  by  the  sun  in  passing  from  a  certain  star 
around  to  that  star  again.     The  sidereal  year  is  21  minutes  longer 
than  the  tropical  year,  and  is,  of  course,  the  true  year,  or  period  of 
the  earth's  revolution  about  the  sun.     But  the  tropical  year  includes 
exactly  the  four  seasons,  and  is,  therefore,  more  convenient.     The  dif- 
ference between  the  two  is  the  result  of  the  precession  of  the  equi- 
noxes (Art.  72). 

2  In  the  Julian  calendar  the  year  is  supposed  to  be  exactly  365J 
days  long.     This  is  too  great,  and  if  a  certain  portion  of  time  is  di« 
vided  into  these  years  there  will  be  fewer  years  than  there  ought  to  be, 
and  the  count  will  fall  behind ;  just  as  when  the  foot-rule  is  too  long 
the  measurement  of  a  board  will  be  too  short. 


108  ASTRONOMY. 


97.  The  Gregorian  Calendar. — In  1582,  when  Greg- 
ory XIII.  was  Pope,  the  calendar  had  fallen  back  10 
days.     In  that  year  the  vernal  equinox  came  on  March 
11,  instead  of  March  21.     As  the  time  of  Easter1  and 
other  festivals  of  the  Catholic  Church  depends  upon  the 
vernal  equinox,  these  festivals  were  gradually  moving 
out  of  their  proper  months.     To  remedy  this,  the  Pope 
introduced  the  Gregorian  Calendar.    This  simply  omitted 
three  of  the  extra  days  every  400  years.     The  equinox 
was  brought  back  to  its  place  in  the  month  by  drop- 
ping 10  days  out  of  the  year  1582,  the  5th  of  October 
being  called  the  15th.     Catholic  countries  adopted  the 
new  calendar  at  once,  but  it  was  not  adopted  in  Eng- 
land until  1752,2  and  in  Russia  the  Old  Style,  as  it  is 
called,  is  still  in  use :   so  that  now  in  Russia  dates 
are  12  days  earlier  than  elsewhere.    The  leap-years  are 
determined  by  the  following  simple  rule.     Every  year, 
except  the  exact  centuries,  that  is  divisible  by  4  is  a  leap-year. 
Every  exact  century  that  is  divisible  by  400  is  a  leap-year. 
Thus,  1884, 1888, 1892,  are  leap-years,  because  they  are 
divisible  by  4 ;  1900  will  not  be  a  leap-year,  because  it 
is  not  divisible  by  400,  but  2000  will  be  a  leap-year. 
This  calendar  loses  only  one  day  in  about  4000  years. 

98.  How  to  Find  what  Day  of  the  Week  a  Given  Day 
will  be. — A  year  of  365  days  makes  52  weeks  and  1 

1  Easter  is  the  Sunday  following  the  first  full  moon  that  occurs  after 
the  vernal  equinox. 

2  By  this  time  the  calendar  was  11  days  behind,  for  the  year  1700 
had  intervened.     By  Act  of  Parliament  in  1752  the  day  after  Septem- 
ber '2  was  called  September  14.     There  was  great  opposition  to  this 
change,  especially  among  the  lower  classes.     They  thought  that  they 
had  been  robbed  of  11  days,  and  ran  after  the  members  of  Parliament 
who  had  secured  the  passage  of  the  law,  pelting  them  with  stones  and 
mud. 


THE  EARTH.  109 


day ;  a  leap-year  makes  52  weeks  and  2  days.  Gen- 
erally, then,  a  given  day  of  the  month  comes  one  day 
later  in  the  week  each  year,  except  when  a  29th  of 
February  has  come  between ;  then  it  comes  two  days 
later.  In  1882  the  4th  of  July  is  on  Tuesday,  in  1883 
on  Wednesday,  but  in  1884  on  Friday. 

99.  Latitude  and  Longitude. — The  latitude   of  any 
place  is  its  distance  in  degrees  north  or  south  of  the 
equator.     The  longitude  of  a  place  is  its  distance  in 
degrees  east  or  west  of  some  fixed  meridian.1     The 
meridian  of  Greenwich2  is  used  more  than  any  other? 
although  different  nations  use  the  meridians  of  their 
capitals  also.     The  location  of  a  place  on  the  earth  is 
always  determined  by  its  latitude  and  longitude.     And 
it  is  absolutely  essential  that  a  ship-captain  should  find 
his  latitude  and  longitude  when  at  sea,  in  order  to  de- 
termine the  course  he  must  take  to  reach  his  destina- 
tion and  avoid  dangers. 

100.  How  to  Find  the  Latitude  of  a  Place.— With  the 
proper  astronomical  instruments,  properly  mounted,  as 
they  are  at  observatories,  it  is  easy  to  determine  lati- 
tude.    The  angular  distance  of  a  star  above  the  hori- 
zon when  it  crosses  the  meridian  is  measured.     As  the 
declination  of  the  star  is  known,  a  simple  arithmetical 


1  What  is  the  latitude  of  a  place  on  the  equator?     On  the  tropic 
of  Cancer?     On  the  Arctic  Circle?     At  the  north  pole?     What  is 
the  longitude  of  the  north  pole,  from  any  meridian  ?     What  is  the 
greatest  possible  latitude  of  anyplace  on  the  earth?     The  greatest 
possible  longitude  ? 

2  Greenwich   (pronounced  grin7!))  is  close  to  London,  and  is  the 
seat  of  the  Koyal  Observatory  of  England.     American  sailors  reckon 
from  the  meridian  of  Greenwich,  and  the  whole  world  ought  to  do  it. 
It  is  mainly  national  pride  that  prevents  it. 

10 


110  ASTRONOMY. 


solution  gives  the  latitude  of  the  place.1  At  sea,  the 
angular  height  of  the  sun  above  the  water  at  noon  is 
measured  with  a  sextant  (Art.  250),  and  from  this  the 
latitude  of  the  ship  is  found  in  the  same  way. 

101.  Longitude  and  Time. — As  the  earth  rotates  once 
on  its  axis  in  24  hours,  every  place  on  the  earth  must 
revolve  around  in  a  circle  in  that  time.     And  as  every 
circle  contains  360°,  in  one  hour  every  place  on  the 
earth  rotates  ^  of  360°,  or  15°.2     This  makes  the  sun 
rise  1  hour  later  for  every  15°  that  a  place  is  west  of  us, 
and  1  hour  earlier  for  every  15°  that  a  place  is  east  of 
us.     If  the  sun  rises  later  upon  places  west  of  us,  of 
course  their  time  is  later  than  ours, — that  is,  their  clocks 
are  behind  or  slower  than  ours.   Places  east  of  us  have 
their  time,  and  therefore  their  clocks,  faster  than  ours. 
The  difference  of  time  is,  of  course,  1  hour  for  every 
15°  that  one  place  is  east  or  west  of  the  other.3 

102.  Longitude  'Found  from  the  Difference  of  Time. — 

1  If  the  star  is  found  to  be  70°  above  the  northern  horizon,  it  must 
foe  90°  —  70°,  or  20°  farther  north  of  the  equator  than  our  zenith,  or 
than  we  are.     Suppose  a  catalogue  of  stars  gives  the  declination  of 
this  star  as  60°  N.     Then,  as  we  are  20°  nearer  the  equator,  our  lati- 
tude must  be  40°  N.     The  declination  of  the  sun  for  every  day  in  the 
year  is  given  in  the  Nautical  Almanac,  for  the  use  of  sailors. 

2  Does  every  part  of  the  earth's  surface  move  through  the  same 
distance  in  24  hours  ?     If  there  is  any  difference,  which  part  of  the 
earth's  surface  moves  fastest  as  the  earth  rotates  ?     Which  slowest? 

8  A  curious  effect  of  this  is  that  messages  sent  westward  by  tele- 
graph seem  to  arrive  at  their  destination  before  they  are  sent.  The 
difference  of  time  between  England  and  New  York  is  about  five  hours. 
After  the  morning  papers  come  out  in  London,  news  from  them  is 
sometimes  telegraphed  to  New  York  by  one  of  the  Atlantic  cables 
and  printed  in  our  papers  the  same  morning.  When  Pope  Pius  IX. 
died  in  1878,  our  American  afternoon  papers  which  were  printed  at 
one  o'clock  announced  that  the  Pope  had  died  at  three  o'clock  the 
same  afternoon. 


THE  EARTH.  HI 


If  one  had  a  watch  that  kept  perfect  time,  he  could  find 
the  longitude  of  any  place  exactly.  He  need  only  set 
his  watch  just  right  at  Greenwich,  and  carry  it  to  the 
place  whose  longitude  is  wanted.  The  difference  of 
time  between  his  watch  and  a  clock  which  gives  the 
correct  time  of  the  place  is  the  difference  of  time  be- 
tween this  place  and  Greenwich.  This  multiplied  by 
15  gives  the  difference  in  degrees,  or  the  longitude  of 
the  place.  If  the  clock  is  faster  than  Greenwich  time, 
the  longitude  of  the  place  is  east ;  if  slower,  the  lon- 
gitude is  west.1  It  is  impossible  to  make  watches  or 
clocks  that  will  keep  perfect  time,  but  clocks  are  made 
which  will  vary  very  little.  Those  made  to  be  carried 
are  called  chronometers,  and  are  always  carried  by  ships 
at  sea.  A  ship's  chronometer  keeps  Greenwich  time 2 
throughout  the  voyage,  and  the  captain  finds  the  cor- 
rect time  at  his  ship  every  clear  day  from  observations 
of  the  sun  with  his  sextant.3  The  difference  between 

1  Greenwich  time  is  5  hrs.  40  sec.  faster  than  Philadelphia  time. 
What  is  the  longitude  of  Philadelphia?  San  Francisco  time  is  3  hrs. 
9  min.  slower  than  Philadelphia  time.  What  is  the  longitude  of  San 
Francisco  ?  The  longitude  of  Pekin  is  116°  27'  east :  what  is  the  dif- 
ference of  time  between  Pekin  and  Philadelphia  ?  When  it  is  noon 
at  Pekin,  what  time  is  it  at  Philadelphia  ? 

3  The  chronometer  need  not  have  Greenwich  time,  but  the  captain 
must  know  how  much  too  fast  or  too  slow  it  is.  The  amount  which  a 
chronometer  gains  or  loses  every  day  is  called  its  rate,  and  is  carefully 
determined  before  going  to  sea.  Allowance  is  made  for  this  in  get- 
ting Greenwich  time  from  the  chronometer. 

s  Besides  the  height  of  the  sun  above  the  horizon  (which  is  taken 
for  this  purpose  about  8  or  9  A.M.  or  3  or  4  P.M.),  the  captain  needs 
his  latitude  (got  as  in  Art.  100)  and  the  sun's  declination  for  that  day 
(given  in  his  Nautical  Almanac).  Knowing  these,  the  time  can  be 
calculated  by  spherical  trigonometry. 

If  the  days  are  cloudy,  but  the  nights  clear,  the  moon  or  certain 
stars  or  planets  may  be  used. 


112  ASTRONOMY. 


this  and  the  chronometer's  time  when  the  observation 
was  made  gives  him  the  ship's  longitude.  Thus  know- 
ing his  latitude  and  longitude,  the  captain  can  find  on 
his  map  just  where  his  ship  is. 

103.  Longitude  Found  by  Telegraph. — When  two  places 
are  connected  by  a  telegraph-line,  this  can  be  used  to 
find  their  difference  of  longitude  in  the  best  and  most 
exact  way.     At  the  time  of  the  passage  of  some  star 
across  the  meridian  of  one  place,  a  signal  is  sent  over 
the  wire  to  the  other  place.    Since  the  electricity  travels 
over  the  wire  at  the  rate  of  about  8000  miles  a  second, 
unless  the  distance  between  the  places  is  great,  the 
signal  reaches  the  second  place  at  practically  the  same 
time  it  started.     If  the  time  when   it  arrives  at  the 
second  place  is  observed,  the  difference  of  time  be- 
tween the  two  places,  and  hence  the  difference  of  longi- 
tude, is  found.     "When  the  distance  is  great  enough  to 
make  the  time  occupied  by  the  passage  of  the  electricity 
perceptible,  a  correction  is  easily  made  for  that. 

104.  Change  of  Days  in  going  around  the  Earth. — If 
a  person  travels  towards  the  west,  each  of  his  days  is 
longer  than  if  he  stays  in  one  place.     (Why  ?)    And  if 
he  travels  entirely  around  the  earth  in  this  direction, 
since  each  of  his  days  has  been  longer,  he  has  not  had 
so  many  of  them,  and  has  had  in  fact  one  day  less  than 
his  neighbors  who  sta}^ed  at  home.     If  he  has  kept  an 
account  of  his  days,  he  will  find  his  reckoning  one  day 
behind  theirs :  what  he  calls  Tuesday  they  call  Wed- 
nesday.    If  he  goes  around  eastward,  he  will  gain  a 
day,  and  his  Tuesday  will  be  Monday  at  his  home.     It 
is  necessary,  then,  to  have  some  line  where  the  day 
changes.     The  line  now  generally  used  is  the  one  just 
opposite  to  the  meridian  of  Greenwich,  180°  from  it 


THE  EARTH.  H3 


either  way.  It  runs  directly  north  and  south,  of  course, 
through  the  Pacific  Ocean,  and  nearly  through  Beh- 
ring's  Strait.1  When  voyagers  from  San  Francisco  to 
Asia  cross  this  line,  they  skip  a  day.  If,  for  instance, 
this  meridian  is  crossed  about  noon  on  Monday,  the 
rest  of  that  day  is  called  Tuesday.  Coming  back,  a 
day  is  repeated  :  Monday  noon  would  suddenly  become 
Sunday  noon,  and  the  next  morning,  Monday  morning 
over  again. 

105.  Refraction. — When  light  passes  from  a  rare  trans- 
parent substance  to  a  dense  transparent  one  (as  from  air 
to  water),  its  course  is  bent,  and  it  becomes  more  nearly 
perpendicular  to  the  surface  of  the  dense  substance.2 
But  when  the  light  is  passing  from  the  dense  to  the 
rare  substance  it  is  bent  in  the  other  direction,  and  be- 
comes more  slanting  to  the  dense  surface.  This  bending 
of  the  light  is  called  refraction.  When  the  end  of  a  tea- 
spoon or  an  oar  is  put  under  water,  the  part  under  water 
seems  to  be  bent  upward;  because  when  the  light 
from  that  part  of  the  spoon  or  oar  comes  out  from  the 
water  into  the  air  it  is  bent  down  a  little,  and,  as  an 

1  The  reckoning  in  the  islands  of  the  Pacific  Ocean  does  not  in  all 
cases  depend  upon  their  position  with  respect   to  this  line.     Those 
which  were  settled  by  voyagers  around  Cape  Horn  had  calendars  one 
day  behind  those  settled  by  voyagers  about  the  Cape  of    Good  Hope, 
without  respect  to  their  situation  as  regards  the  180th  meridian.     In 
some  cases  this  difference  still  exists.     When  our  government  bought 
Alaska  the  reckoning  there  was  one  day  ahead  of  ours. 

When  it  is  9  o'clock  A.M.,  Wednesday,  at  St.  Louis  (90°  15'  W.), 
over  what  part  of  the  earth  is  it  Wednesday,  and  what  day  is  it  over 
the  rest  of  the  earth  ? 

Ans.—  From  134°  W  E.  of  Greenwich  to  180°  W.  it  is  Wednesday. 
Over  the  rest  of  the  earth  it  is  Thursday  (according  to  Art.  104). 

2  If  the   light  comes  down  perpendicular  to  the  dense  surface  it 
cannot  become  more  perpendicular,  and  so  it  is  not  bent  at  all. 

10* 


114 


ASTRONOMY. 


object  always  seems  to  be  in  the  direction  in  which 
the  light  comes  to  the  eye,  the  part  under  water  seems 
to  be  higher  than  it  really  is.  Refraction  is  fully  ex- 
plained in  Natural  Philosophy,  and  various  experiments 
and  illustrations  of  it  will  be  found  there. 

106.  Refraction  of  the  Heavenly  Bodies  by  the  Air. — 
The  lower  part  of  the  air  is  denser  than  the  upper  part, 
and  the  light  from  the  heavenly  bodies  is  consequently 
refracted  by  coming  through  the  air,  and  they  appear 
to  be  higher  up  in  the  sky  than  they  really  are.  Fig. 
25  illustrates  this.  0  is  the  position  of  the  observer, 


FIG.  25.— THE  APPARENT  ELEVATION  OF  A  STAR  BY  ATMOSPHERIC  REFRACTION. 
(Greatly  exaggerated.) 

and  Z  his  zenith.  The  curved  lines  represent  the  at- 
mosphere. The  lower  ones  are  closer  together,  indi- 
cating that  the  atmosphere  there  is  denser,  while  the 
higher  part  of  the  atmosphere  is  much  rarer,  as  the 
lines  indicate.  The  true  position  of  the  star  is  at  S, 
but  its  light  in  passing  through  the  air  is  bent  down,  as 
shown  in  the  figure,  and  the  star  seems  to  be  at  S',  above 


THE  EARTH.  H5 


its  true  place.  As  the  atmosphere  gradually  grows 
denser,  the  path  of  the  light  through  it  is  a  continued 
curve  down  to  the  observer's  eye,  as  shown  in  the  figure. 
The  amount  of  the  refraction  is  not  truly  represented, 
but  greatly  exaggerated,  to  show  it  more  clearly.  Re- 
fraction is  greatest  when  the  heavenly  body  is  at  the 
horizon,  for  its  light  is  then  most  inclined,  and  there- 
fore most  bent  out  of  its  course.  It  is  then  over  half 
a  degree  (35'),  but  decreases  fast  at  first,  then  slowly, 
until  in  the  zenith  there  is  no  refraction  (note  2,  p.  113). 
In  making  observations  on  the  height  of  heavenly 
bodies,  astronomers  must  always  correct  for  refraction. 
That  is,  they  must  subtract  (Why  ?)  from  the  apparent 
height  the  amount  of  refraction  for  that  height.  This 
is  found  from  tables  made  for  the  purpose. 

107.  Curious  Effects  of  He/motion. — Since  refraction 
elevates  heavenly  bodies  more  than  J°  when  they  are 
at  the  horizon,  and  the  sun  and  moon  are  each  about 
J°   in   diameter,  these   two   bodies   when    rising   and 
setting  seem  to  be  just  above  the  horizon  when  they 
are  really  just  below  it.     This  makes  the  sun  (or  moon) 
rise  three  or  four  minutes  earlier,  and  set  three  or  four 
minutes  later,  than  it  would  if  there  were  no  refraction, 
thus  adding  six  or  eight  minutes  to  the  length  of  the 
day. 

When  just  above  the  horizon,  the  sun  and  moon — 
especially  the  latter — are  sometimes  seen  to  be  flattened, 
the  vertical  diameter  being  shorter  than  the  horizontal 
one.  This  is  also  due  to  refraction.  Because  the  lower 
edge  of  the  sun  or  moon  is  nearer  the  horizon,  it  is 
elevated  by  refraction  more  than  the  upper  edge,  thus 
causing  the  flattening. 

108.  Why  the  Sun  and  Moon  appear  Largest  when 


116  ASTRONOMY. 


Rising  and  Setting. — The  apparent  enlargement  of  the 
sun  and  moon  when  rising  and  setting  is  sometimes 
attributed  to  refraction ;  but  this  is  a  mistake.  This 
enlargement  is  an  optical  delusion.  When  the  sun 
and  moon  are  near  the  horizon  they  seem  larger,  be- 
cause the  long  stretch  of  country  between  gives  us  a 
better  appreciation  of  their  great  distance  from  us. 
Every  one  knows  from  his  own  experience  that  we 
habitually  judge  of  the  size  of  objects  from  their  known 
or  suspected  distance.  Besides,  near  the  horizon  we 
can  compare  the  sun  and  moon  with  objects  whose  size 
we  know,  as  fences,  trees,  houses,  and  the  like.  If 
they  are  looked  at  through  a  tube, — a  roll  of  paper,  for 
instance, — the  illusion  will  disappear.  And  if  carefully 
measured,  the  moon's  diameter  is  found  to  be  really 
less  at  the  horizon  than  when  overhead,  for  at  the  hori- 
zon it  is  farther  from  us.1 

109.  Twilight. — After  the  sun  has  set  upon  us  at  the 
surface  of  the  earth  it  still  shines  for  a  while  upon  the 
clouds  and  air  above  us.  The  reflection  (not  refraction] 
of  this  light  causes  twilight.  The  same  cause  gives  us 
twilight  in  the  morning.  As  the  sun  sinks  lower  and 
lower,  less  of  our  sky  is  lighted  up,  and  the  evening 
twilight  gradually  fades  away.  Observation  shows 
that  it  does  not  entirely  disappear  until  the  sun  is  about 
18°  below  the  horizon.  Near  the  equator  twilight  is 
short,  for  there  the  sun  always  goes  down  nearly  per- 

1  When  rising,  the  moon  is  farther  by  the  length  of  the  earth's  ra- 
dius (How  many  miles  ?)  from  us  than  when  it  is  overhead.  Let  the 
student  draw  a  figure  of  the  earth  with  the  moon  upon  the  horizon, 
and  also  overhead,  and  explain  this  clearly.  The  sun  is  so  far  away 
that  the  difference  between  its  morning  and  its  noon  diameter  could 
not  be  detected  by  measurement. 


THE  EARTH.  117 


pendicular  to  the  horizon.  But  in  the  temperate  and 
frigid  zones  the  sun  always  moves  obliquely  to  the 
horizon  at  sunset  and  sunrise,  and  must  move  more 
than  18°  in  its  path  to  go  18°  below  the  horizon.  The 
nearer  we  go  to  the  poles,  the  more  oblique  is  the  sun's 
motion,  and  the  longer  twilight  lasts.1  In  Northern 
Europe  twilight  in  midsummer  lasts  all  night,  and  at 
the  north  pole  it  lasts  more  than  two  months. 

110.  The  Aurora  Borealis. — The  aurora  borealis,2  or 
simply  the  aurora,  is  in  some  years  a  frequent  phe- 
nomenon in  our  northern  skies.  It  commonly  consists 
of  rays  of  light,  sometimes  of  a  reddish  tinge,  extend- 
ing up  above  the  northern  horizon.  It  is  more  com- 
mon the  farther  north  one  goes.  But  it  is  more  fre- 
quently seen  around  the  Arctic  Circle  and  the  magnetic 
pole  than  near  the  north  pole  itself.  Many  attempts 
have  been  made  to  measure  the  height  of  the  aurora, 
and  the  results  vary  from  a  few  thousand  feet  to  five  or 
six  hundred  miles. 

How  the  aurora  is  produced  is  not  yet  known.  It  is 
probably  caused  in  some  way  by  electricity,  for  auroral 
displays  are  very  frequently  accompanied  by  electric 
storms  upon  the  earth;  strong  currents  of  electricity 

1  This  may  be  very  clearly  shown  with  any  globe  which  has  a  hori- 
zon (a  horizontal  ring  about  the  middle).     For  a  place  in  the  north- 
ern hemisphere,  slide  the  globe  around  until  the  north  pole  is  as  many 
degrees  above  the  horizontal  ring  as  the  place  is  north  of  the  equator. 
Now  mark  with  chalk  the  probable  place  of  the  sun  at  that  time  of 
year  (on  a  celestial  globe  the  place  is  marked),  and  revolve  the  globe. 
(Which  way?)     If  the  place  is  far  north,  the  pole  will  be  near  the 
horizon,  and  the  chalk-mark  will  be  seen  to  turn  through  a  long  arc 
before  it  is  as  much  as  18°  below  the  horizon. 

2  AurO'ra   borea/lis,  from   Latin  aurora,  daybreak,  and  borealis, 
northern. 


118  ASTRONOMY. 


pass  along  telegraph-wires,  and  compass-needles  are 
much  disturbed.  And,  besides,  if  electricity  be  allowed 
to  pass  through  "a  long  glass  tube  from  which  the  air 
has  been  almost  exhausted  with  an  air-pump,  an  ap- 
pearance very  much  like  the  aurora  is  produced. 

At  certain  periods  displays  of  the  aurora  are  unusu- 
ally frequent  and  brilliant.  As  has  been  said  in  Art. 
42,  these  periods  seem  to  occur  about  every  eleven 
years,  and  at  the  same  times  as  the  periods  of  numer- 
ous sun-spots,  with  which  they  are  probably  in  some 
unknown  way  connected.  For  several  years  before 
1881  the  auroras  were  very  infrequent,  but  during  1881, 
1882,  and  1883  they  increased  in  number  and  brilliancy, 
and  then  the  number  gradually  diminished  for  several 
succeeding  years.  Records  and  descriptions  of  auroral 
displays  would  be  well  worth  making. 

/I' 

THE  TIDES. 

111.  What  the  Tides  are. — Every  one  who  has  spent 
even  a  short  time  on  the  sea-coast,  on  a  bay,  or  near  the 
mouth  of  a  river,  has  noticed  that  every  day  for  about 
six  hours  the  water  slowly  rises  (flood  tide)  until  it  is 
several  feet  deeper,  and  then  as  slowly  fsd\s(ebb  tide)  for 
the  next  six  hours.     The  same  thing  is  repeated  in  the 
night.     These  ^  risings  of  the  waters  of  the  ocean  are 
called  tides,     "they  are  caused  by  two  waves  which  are 
constantly  passing  around  the  earth  from  east  to  west, 
opposite  to  the  direction  of  the  earth's  rotation.     They 
are  not  exactly  twelve  hours,  but  nearly  twelve  and  one- 
half  hours  apart,  so  that  each  of  the  two  tides  comes 
about  one  hour  later  every  day. 

112.  Tides  caused  mainly  by  the  Moon. — If  the  matter 


THE  EARTH.  H9 


be  looked  into  a  little  more  closely,  it  will  be  noticed 
that  high  tide  always  comes  when  the  moon  is  about 
the  same  place  in  the  sky.1  The  moon  rises  about  one 
hour  later  every  night,  and  we  have  seen  that  high  tide 
comes  about  one  hour  later  every  day.  This  remark- 
able connection  between  the  moon  and  the  tides  was 
noticed  in  very  ancient  times,  and  men  knew  that  the 
moon  in  some  way  caused  the  tides,  long  before  they 
could  explain  how  it  caused  them.  Sir  Isaac  Newton 
was  the  first  to  show  just  how  they  were  caused  by  the 
moon. 

113.  How  the  Moon  causes  the  Tides. — To  show  ex- 
actly how  the  moon  causes  the  tides  requires  a  difficult 
mathematical  demonstration.  But  the  common  expla- 
nation is  illustrated  by  Fig.  26.  Because  the  water 


FIG.  26.— THE  TIDES.    High  tide  at  0  and  D.    Low  tide  at  the  two  points  half-way  be- 
tween,  immediately  in  front  of  and  behind  the  middle  of  the  figure. 

on  the  side  of  the  earth  nearest  to  the  moon  is  more 
strongly  attracted  by  the  moon  than  the  earth  itself  is 
attracted  by  it,  the  water  is  heaped  up  on  that  side, 
forming  the  direct  tide,  at  D  in  the  figure.  And  because 

1  This  is  true  of  either  of  the  two  daily  tides,  although  only  one  of 
*hem  would  occur  while  the  moon  was  above  our  horizon.  At  New 
York  high  tide  always  occurs  when  the  moon  is  about  southeast ;  at 
New  Castle,  Del.,  when  the  moon  is  south }  and  at  Baltimore  when 
the  moon  is  rising  and  setting.  Why  these  intervals  differ  at  differ- 
ent places  is  explained  in  Art.  115. 


120  ASTRONOMY. 


the  moon  attracts  the  earth  itself  more  strongly  than  it 
attracts  the  water  on  the  opposite  side  of  the  earth,  it 
pulls  the  earth  away  from  the  water  there,  leaving  it 
heaped  up  on  the  opposite  side,  and  forming  the  oppo- 
site tide  there,  at  0  in  the  figure.  This  explanation 
shows  pretty  clearly  how  the  direct  tide  is  formed ;  but 
most  persons  cannot  see  how  the  earth  can  be  pulled 
away  so  as  to  leave  the  opposite  tide  behind  it  and  yet 
approach  no  nearer  to  the  moon.  The  following  ex- 
planation may  help  to  make  the  cause  of  the  opposite 
tide  clear. 

114.  How  the  Opposite  Tide  is  produced.— We  are  accus- 
tomed to  say  that  the  moon  revolves  about  the  earth. 
But  it  is  proved  in  Natural  Philosophy  that  it  is  impos- 
sible for  one  body  to  stand  still  while  another  revolves 
about  it  in  this  way.  They  must  both  revolve  about  their 
common  centre  of  gravity.1  The  centre  of  gravity  of  the 
earth  and  moon  is  within  the  earth,  about  three-fourths 
of  the  way  from  the  centre  to  the  surface,  at  C  in  the 
figure,  so  that  the  earth  as  well  as  the  moon  is  really 
revolving  about  this  point  C.  As  the  earth  revolves 
around  this  point,  the  water  at  0,  being  attracted 
less  strongly  by  the  moon,  swings  out  into  a  little  larger 
circle.  This  heaps  up  the  water  there,  and  always 
makes  a  tide  on  the  opposite  side  from  the  moon.  If 
a  hollow  india-rubber  ball  be  pulled  on  its  two  opposite 
sides  by  two  strings,  it  will  take  this  spheroidal  shape. 
The  opposite  forces  in  the  case  of  the  earth  are  the 
moon's  attraction  and  centrifugal  force.  The  land  is 
solid  and  cannot  be  pulled  out  of  shape,  but  the  water 


1  Tiie  centre  of  gravity  of  two  bodies  is  the  point  about  which  they 
would  balance  each  other. 


THE  EARTH.  121 


yields.  The  revolution  of  the  earth  and  moon  around 
their  common  centre  of  gravity  takes  about  a  month 
(27J  days).  The  reason  we  have  a  tide  about  every 
twelve  hours  is  that  the  earth  in  rotating  on  its  axis 
turns  under  these  two  projections  and  carries  us  around 
to  them.  It  is  this  turning  of  the  earth  under  these  two 
projections  that  causes  the  tidal  friction  which  it  is 
thought  may  be  slowly  retarding  the  earth's  rotation 
(Art.  68).  This  also  explains  the  fact  that  the  main 
motion  of  the  tidal  waves  is  from  east  to  west,  for  we 
are  carried  towards  the  east  to  them. 

115.  The  Sun's  Influence  upon  the  Tides.— Although 
the  sun  is  so  much  farther  than  the  moon  from  the 
earth,  yet  its  prodigious  mass  makes  its  attraction  far 
greater  upon  the  earth  than  the  moon's  attraction.  But 
the  power  to  raise  a  tide  depends  not  so  much  upon  the 
strength  of  the  attracting  force  as  upon  the  difference  of 
its  attractions  upon  the  opposite  sides  of  the  earth. 
The  sun  is  so  far  away  that  it  draws  the  opposite  side 
of  the  earth  almost  as  strongly  as  the  near  side,  so  that 
it  does  not  draw  up  a  high  direct  tide,  nor  does  it  draw 
the  earth  much  away  from  the  opposite  waters  to  raise 
an  opposite  tide.  Yet  the  sun's  influence  is  perceptible. 
The  sun  can  raise  tides  about  two-fifths  of  the  height  of 
the  moon's  tides,  but  these  are  generally  combined  with 
those  raised  by  the  moon.  If,  in  Fig.  26,  the  sun  were 
to  the  right  of  M  (new  moon),  the  sun's  tides  would  be 
piled  upon  the  moon's  tides,  which  would  make  them 
unusually  high.  The  same  result  would  follow  if  the 
sun  were  opposite  to  the  moon  (full  moon).  These  are 
the  spring  tides,  and  occur  every  two  weeks.  But  if 
the  sun  and  moon  are  90°  apart,  as  when  the  sun  is 
directly  in  front  of  or  behind  the  earth,  in  Fig.  26, 

il 


122  ASTRONOMY. 


they  pull  the  water  in  opposing  directions.  Then  the 
sun  lowers  the  moon's  tides  :  these  are  called  neap  tides. 
The  difference  between  spring  and  neap  tides  at  New 
York  is  about  two  feet. 

116.  The  Land  modifies  the  Action  of  tfie  Tides.— If  the 
whole  earth  were  covered  with  water  of  the  same  depth, 
the  tides  would  pass  regularly  around  the  earth  ;l  but 
continents  and  shallow  water  greatly  modify  the  action 
of  the  tides.    Since  the  Antarctic  Ocean  is  the  only  one 
extending  around  the  earth,  the  tidal  waves  are  be- 
lieved to  originate  there.     Branches  of  these  run  up 
the  great  oceans  opening  into  the  Antarctic,  and  these 
run  into  our  harbors  and  bays,  causing  the  tides  there. 
As  the  tide  at  the  mouth  of  a  river  rises,  it  presently 
becomes  higher  than  the  water  farther  up  the  river, 
and,  as  it  must  flow  down-hill,  the  water  rushes  up  the 
river  and  causes  the  tides  there.     Along  the  lower  part 
of  rivers  emptying  into  the  ocean,  while  the  tide  is 
rising  a  current  runs  up  the  river.     At  high  tide  the 
current  turns  and  begins  to  run  down  again  towards 
the. sea.     But  for  some  distance  from  the  mouth  the 
current  runs  down  for  more  than  half  ofjhs  twelve 
hours,  for  it  is  some  time  after  high  tide  begins  before 
the  water  rises  to  the  ordinary  height  of  the  river  there. 

117.  Effect  of  Bays  upon  the  Height  of  the  Tide.— If  a 
bay  has  a  broad  mouth  opening  in  the  direction  of  the 

1  In  the  deep  waters  of  the  ocean  the  tide-wave,  like  other  water- 
waves,  moves  forward  by  the  rising  and  falling  of  the  particles  of 
water,  and  not  by  their  moving  forward.  The  motion  of  each  par- 
ticle forward  and  backward  is  very  slight;  just  as  when  a  breeze 
sweeps  across  a  wheat-field,  a  wave  passes  swiftly  over  the  field,  but 
each  head  of  wheat  only  bends  over  and  rises  again.  Near  a  shore, 
where  the  water  grows  shallow,  the  tide  may  have  considerable  for- 
ward motion. 


THE  EARTH.  123 


tidal  wave,  and  gradually  becomes  narrow  towards  its 
upper  end,  it  acts  like  a  funnel,  and  the  water  may 
be  forced  up  to  a  great  height  there.  This  accounts 
for  the  different  heights  of  the  tide  at  various  places 
along  the  coast.  In  mid-ocean  the  average  height  of 
the  tides  is  about  three  and  a  half  feet.  At  New  York 
it  is  four  and  a  half  feet;  at  Boston,  more  than  twice 
as  great.  At  the  upper  end  of  the  Bay  of  Fundy,  and 
in  the  English  Channel,  the  tides  sometimes  rise  to  a 
height  of  seventy  feet. 

118.  Tides  on  Inland  Seas  and  Lakes. — Seas  which 
have  little  or  no  communication  with  the  ocean  have 
very  little  tide.  On  the  Mediterranean,  much  the 
largest  of  these  seas,  there  is  a  tide  about  eighteen 
inches  high,  which  must  be  raised  upon  the  sea  itself, 
for  the  narrow  Strait  of  Gibraltar  allows  the  ocean 
tides  to  affect  but  a  small  part  of  the  sea.  On  the 
great  American  lakes  a  tide  one  or  two  inches  high 
has  been  detected. 

NOTE  :  Change  of  Latitude. — Recent  investigations  show  that  the 
latitudes  of  places  on  the  earth's  surface  are  subject  to  a  change 
amounting  to  nearly  a  half-second  of  arc.  This  is  due  to  an  oscilla- 
tion of  the  axis  of  the  earth,  by  which  the  pole  describes  a  circle  of 
fifty  or  sixty  feet  in  diameter  in  about  fourteen  months.  Hence  the 
equator  is  continually  shifting  its  place  to  a  slight  extent. 


124  ASTRONOMY. 


CHAPTER  V. 

THE  MOON.       D 

Distance  from  the  Earth,  240,000  Miles.  Diametgr^  2,160 
Miles.  Length  of  Year,  the  same  as  that  of  the  Earth. 
Length  of  Day,  29£  Days.  Specific  Gravity,  3J. 

119.  The  Moon  a  Satellite  of  the  Earth.— The  moon 
revolves  about  the  earth1  from  west  to  east,^ist  as  the 
earth  revolves  about  the  sun,  but  makes  a  revolution 
every  27J  daysx  Every  one  must  have  noticed  this 
motion  of  the  moon  among  the  stars  towards  the  east 
from  night  to  night.2  The  new  moon  is  first  seen  in  the 
early  evening,  low  in  the  west.  At  the  same  hour  the 
next  evening  it  is  higher  up  in  the  sky :  it  has  moved 
eastward  among  the  stars  since  the  night  before.  The  next 
evening  it  is  higher  still,  and  in  two  weeks  it  is  in  the 
opposite  side  of  the  heavens,  and  is  seen  in  the  early 
evening  just  rising  in  the  east.  This  motion  of  the 
moon  causes  it  to  rise  nearly  an  hour  later  every  night, 
— a  familiar  fact,  and  one  often  useful  in  determining 
off-hand  a  few  days  in  advance  how  early  in  the  even- 
ing there  will  be  moonlight. 


1  Keally  both  earth  and  moon  revolve  about  their  common  centre  of 
gravity  (Art.  113),  but,  as  their  centre  of  gravity  is  inside  of  the  earth, 
the  statement  here  is  correct. 

2,Be  careful  again  to  distinguish  this  motion  from  the  nightly 
motion  from  east  to  west  caused  by  the  earth's  rotation. 


THE  MOON.  125 


120.  The  Moon's  Orbit  around  the  Earth. — The  moon's 
orbit  around  the  earth,  like  the  orbits  of  all  the  planets 
around  the  sun,  is  an  ellipse.     Its  eccentricity  (Art.  25) 
is  about  -^ :  four  times  that  of  the  earth's  orbit.    This 
makes  the  moon's  distance  from  the  earth  vary  nearly 
40,000  miles ;  but  the  eye  could  not  distinguish  its  orbit 
from  a  circle,  although  it  could  easily  see  that  the  earth 
is  not  in  the  centre  of  the  orbit.      The  point  in  the 
moon's  orbit  which  is  nearest  to  the  earth  is  called  the 
perigee  ;*  the  farthest  point  is  the  apogee.2 

121.  The  Moon's  Path  around  the  Sun.— While  the 
moon  is  revolving  about  the  earth,  the  earth  itself  is 
moving  forward  more  than  1J  millions  of  miles  a  day 
in   its   own   orbit  around  the  sun.     This  makes  the 
moon's  real  .path^a  waving  line,  crossing  the  earth's 
orbit  backwards  and  forwards.     Fig.  27  shows  this. 


FIG.  27.— THE  DOTTED  LINE,   PART  OF  THE  EARTH'S  PATH  AROUND  THE  SUN.     THB 
FULL  LINE,  PART  OF  THE  MOON'S  PATH  AROUND  THE  SUN. 

When  the  earth  is  at  E,  the  moon  is  just  in  front  of  it, 
at  M.  But  the  earth  by  its  attraction  holds  the  moon 
back,  and  gradually  gains  upon  it,  until,  one  week  later, 
at  E2  and  M2,  they  are  side  by  side.  The  earth's 

1  Per'i-gee,  from  Greek  peri,  near,  and  ge,  the  earth. 

2  Ap/o-gee,  from  Greek  apo,  from,  and  ge,  the  earth.     (G  is  soft 
in  both  words.)    What  points  of  the  earth's  orbit  correspond  to  these ? 

II* 


126  ASTRONOMY. 

swifter  motion  carries  it  on  past  J:he  moon,  and  in 
another  week  it  is  at  E4,  just  in  front  of  the  moon.  In 
these  two  weeks  the  moon  seems  to  us  to  have  made 
half  a  revolution  around  the  earth ;  really  it  has  moved 
along  the  curve  MM4,  inside  of  the  earth's  path,  and 
somewhat  slower  than  the  earth,  so  that  from  being 
immediately  in  front  of  the  earth  it  has  fallen  back 
and  is  now  right  behind  it.  Here  it  crosses  the  earth's 
path,  and,  pulled  on  by  the  earth's  attraction,  gains 
upon  it,  and  in  another  week  is  beside  it  again,  at 
M6;  and  at  the  end  of  the  month  it  is  immediately 
in  front  of  the  earth  again,  and  crosses  its  path.  It  has 
now,  as  it  seems  to  us,  completed  a  revolution  around 
the  earth,  and  so  it  has.  It  is  just  as  if  a  person  while 
on  the  deck  of  a  fast-sailing  ship  should  walk  slowly 
around  the  mast.  He  really  does  walk  around  the 
mast,  but  on  account  of  the  swifter  motion  of  the  ship 
his  path  over  the  surface  of  the  earth  is  a  wavy  line 
crossing  the  path  of  the  ship's  mast. 

The  figure  necessarily  exaggerates  the  amount  of  this 
wavy  motion  :  so  small  is  it  in  comparison  with  the  size 
of  the  earth's  orbit,  that  the  moon's  path  could  scarcely 
be  distinguished  from  a  perfect  ellipse.  And  if  the 
earth  were  suddenly  destroyed,  the  moon  would  keep 
on  in  a  perfect  ellipse  about  the  sun,  which  could  not 
easily  be  distinguished  from  its  present  path. 

122.  The  Moon's  Phases. — The  most  noticeable  fea- 
ture of  the  moon  is  its  varying  phases,  from  the  thinnest 
crescent  to  the  round  full  moon,  and  then  back  to  the 
crescent  again.  To  understand  these^ phases,  it  mus_tJbe 
remembered  that  the  moon  shines  only  by  reflecting, 
sunlight,  and  therefore  the  hal?  oflhe  moon  which  is 
turnecTtowards  the  sunjsalways  bright,  while  the  other 


THE  MOON.  127 


half  is  always  dark.  When  the  moon  passes  between 
the  earth  and  the  sun,1  its  dark  side  is  turned  towards 
us.  This  is  new  moon,  but  we  cannot  see  it  then.  A 
day  or  two  later,  it  has  moved  to  one  side  of  the  line 
between  the  sun  and  the  earth,  so  that  we  can  see  a 
narrow  strip  of  the  bright  side  of  the  moon  looking 
like  a  thin  crescent.2  We  call  this  the  new  moon ;  but 
the  exact  time  of  new  moon  as  given  in  the  almanac 
was  when  the  sun  and  moon  were  in  conjunction  (Art. 
24).  In  a  week  the  moon  is  around  beside  the  earth,  so 
that  of  the  side  turned  towards  us  half  is  light  and  half 
is  dark.  The  moon  is  half  full :  it  is  at  the  .first,  quarter. 
Then  it  becomes  gibbous,  or  more  than  half  full ;  in  a 
week  from  half  full  U  is  on  the  opposite  side  of  the  earth 
from  the  sun,  and  we  see  the  whole  surface  lighted  up  by 
the  sun  :  it  is  full  moon.3  In  another  week  the  moon  is 
beside  us  againj  it  is  again  half  full,  and  is  at  the  third 
quarter.  As  it  goes  around  towards  the  sun,  more  and 
more  of  its  dark  side  turns  towards  us,  and  the  crescent 
grows  narrower  and  narrower,  until  it  disappears  at 
new  moon  again. 

Fig.  28  illustrates  these  changes.     The  sun  is  sup- 

1  The  moon  does  not  generally  pass  directly  between  the  earth  and 
the  sun.  but  a  little  above  or  a  little  below  it.     When  it  does  pass 
directly  between,  we  have  an  eclipse  of  the  sun.     Nor  at  full  moon 
are  the  three  bodies  generally  in  a  straight  line.     When  they  are, 
the  moon  is  eclipsed. 

2  The  horns  of  the  new  and  of  the  old  moon  always  point  away 
from  the  sun.     Do  you  see  why  ? 

There  is  a  wide-spread  superstition  that  if  the  horns  of  the  new 
moon  point  upward  there  will  be  dry  weather,  because  the  moon 
holds  the  water,  but  that  if  the  horns  point  downward  there  will  soon 
be  rain.  What  do  you  think  about  it  ?  (See  Art.  135.) 

3  What  time  of  day,  and  where  in  the  sky,  do  we  see  the  new  moon  ? 
the  full  moon  ?  the  old  moon  ?  Why  ? 


128  ASTRONOMY. 

posed  to  be  above  the  figure.     The  figures  upon  the 
circumference  of  the  circle  represent  the  moon  in  its 


Fio.  28.— THE  MOON'S  PHASES. 


different  positions :  the  half  turned  towards  the  sun  will 
be  noticed  to  be  always  bright.  The  outside  figures 
show  the  different  phases. 


THE  MOON.  129 


123.  Time  in  which  the  Moon  Revolves  around   the 
Earth. — The  moon  makes  a  revolution  around  the  earth 
in  27  J  days.     That  is,  if  the  moon  passes  a  certain  star1 
at  twelve  o'clock  to-night,  it  will  pass  the  same  star 
again  27  J  days  afterwards.    But  if  the  moon  passes  the 
sun  at  a  certain  time  (new  moon),  when  it  comes  around 
to  the  same  place  the  sun  has  moved  forward  in  its  ap- 
parent yearly  motion  around  the  earth,  and  it  takes  the 
moon  more  than  two  days  longer  to  catch  the  sun.    So 
that  the  time  from  new  moon  to  new  moon  is  29J  days. 
This  is  called  a  lunation?  or  lunar1  month..    It  is  the 
foundation  of  our  months. 

124.  The  Same  Side  of  the  Moon  always  turned  towards 
the  Earth. — A  little  attention  shows  that  we.  always  see 
the  same  side  of  the  moon.     The  well-known  dark 
markings  upon'  it  are  always  seen  when  enough  of  the 
side  next  us  is  lighted  up  to  show  them.     Closer  ex- 
amination with  the  telescope  proves  the  same  thing. 
The  moon  must  therefore  turn  upon  its  axis  just  as  fast 
as  it  revolves  around  the  earth,2  or  in  27J  days.     This 
is  the  moon's  sidereal^  day.     Its  solar  or  real  day  is  the 
time  from  new  moon  until  new  moon  again,  29  J  days. 
Daylight  and  night  on  the  moon  are  each,  then,  nearly 
fifteen  days  in  length. 

A  curious  and  quite  probable  cause  of  this  remark- 
able motion  of  the  moon  is  this.  The  present  appear- 
ance of  the  moon's  surface  gives  strong  evidence  of 
the  fact  that  the  moon  was  once  at  least  partly  liquid 
or  molten.  The  earth's  attraction  must  then  have 

1  From  Latin  luna,  the  moon. 

*  Let  the  student  walk  around  a  table,  always  keeping  his  face 
towards  it,  and  he  will  find  that  his  body  has  turned  completely 
around,  so  that  he  has  faced  every  side  of  the  room  in  succession. 


13(y  ASTRONOMY. 


raised  enormous  tides  in  this  molten  surface,  and 
the  friction  of  these  tidal  waves  may  have  retarded 
its  rotation  until  it  turned  upon  its  axis  just  as  it  re- 
volved about  the  earth.  We  have  already  learned  that 
the  tides  upon  the  earth  may  be  retarding  its  rotation 
slightly. 

125.  The  Moon's  Librations. — Although  the  moon 
always  keeps  the  same  side  turned  towards  the  earth, 
still  there  are  some  small  oscillations  that  allow  us 
to  see  a  small  part  of  the  other  side  along  the  edge 
These  oscillations  are  called  libratiojis.1  There  are  three 
of  these.  The  moon  turns  on  its  axis  uniformly,  but 
since  its  path  around  the  earth  is  an  ellipse,  its  motion 
in  that  path  is  not  uniform  (Art.  29).  On  this  account 
the  moon  sometimes  turns  on  its  axis  a  little  faster  or 
a  little  slower  than  it  revolves  about  us,  and  allows  us 
to  see  somewhat  farther  than  usual  alone;  its  equator. 
If  the  moon's  path  were  a  circle,  there  would  be  no 
such  libration  as  this. 

A  second  libration  is  due  to  the  fact  that  the  moon's 
axis  is  not  quite  perpendicular  to  the  plane  of  its  orbit, 
and  its  poles  therefore  lean  towards  the  earth  alter- 
nately, just  as  the  earth's  poles  lean  towards  the  sun 
to  produce  summer  and  winter.  This  allows  us  to  see 
at  every  revolution  of  the  moon  a  little  way  beyond  its 
poles. 

And,  finally,  two  observers  on  opposite  sides  of  the 
earth  can  each'  see  farther  around  the  moon  in  his  di- 
rection than  the  other  can,  so  that  both  together  see 
more  than  half  of  the  moon's  surface.  This  is  the 
third  libration. 

1  From  Latin  libratio,  a  balancing. 


THE  MOON.  131 


These  three  librations  together  enable  us  to  see  at 
various  times  nearly  one-fifth  of  the  opposite  side  of 
the  moon,  so  that  in  all  about  six- tenths  of  the  moon's 
surface  can  be  seen  by  us. 

126.  The  Harvest  Moon. — A  celestial  globe  shows 
very  clearly  that  near  the  autumnal  equinox  (Septem- 
ber 22)  the  ecliptic  (Art.  30)  is  most  inclined  to  the 
horizon — that  is,  it  makes  the  smallest  possible  angle 
with  it — about  sunset.  And  it  is  also  about  sunset 


FIG.  29. — THE  HARVEST 


that  the  full  moon  rises.  The  figure  shows  the  full 
moon  just  on  the"  horizon  at  sunset  on  September 
22.  At  the  time  of  sunset  on  September  23  the  moon 
has  moved  its  regular_daily  journey  .of  13°  along  the 
ecliptic1  to  the  position  shown  in  the  figure.  It  is  now 
below  the  horizon,  but,  because  the  ecliptic  makes  so 

1  The  moon's  path  does  not  quite  coincide  with  the  ecliptic,  but 
does  so  very  nearly,  never  being  more  than  about  5°  from  it. 


1S2  ASTRONOMY. 

small  an  angle  with  the  horizon,  the  moon  is  but  a 
short  distance  below  it,  and  will  rise  at  a  only  a  few 
minutes  later  than  on  the  night  before.  By  the  next 
evening  the  moon  is  another  13°  farther  along  on 
the  ecliptic,  but  still  it  is  not  far  below  the  horizon, 
and  will  rise  at  6  a  few  minutes  later  again.  The  next 
night  it  will  rise  at  c  a  few  minutes  later  still.  And  so 
the  full  moon  which  comes  on,  or  nearest  to,  the  au- 
tumnal equinox  rises  only  a  few  minutes  later  evening 
after  evening  for  several  days.1  This  is  called  ttr  har- 
vest moon,  because  in  England  harvest  comes  at  tMs 
time,  and  the  moon  being  about  full,  and  rising  night 
after  night  nearly  at  the  same  time,  helps  with  its  light 
in  the  gathering  of  the  harvest.  The  harvest  moon  is 
much  more  noticeable  in  England  and  other  high  lati- 
tudes than  here.  In  Edinburgh,  if  the  moon  rose  at 
six  o'clock  on  September  22,  it  would  rise  only  fifteen 
minutes  later  the  next  night,  and  only  fifteen  min- 
utes later  again  on  the  next.  In  the  northern  United 
States  the  harvest  moon  rises  about  half  an  hour  later 
each  night. 

127.  Observation  of  the  Moon. — To  the  naked  eye  the 
moon's  surface  shows  only  the  dark  patches  which 
vivid  imaginations  sometimes  call  "the  njan  in  the 
moon."  These  patches  are  simply  parts  of  the  moon's 
surface  which  are  darker  than  the  rest.  But  through 
a  telescope  its  appearance  is  very  different.  The  moon 

1  The  figure  will  probably  make  the  explanation  of  the  harvest 
moon  clear  to  the  student,  but  a  globe  having  the  ecliptic  marked 
upon  it  is  better  than  any  figure.  If  the  successive  daily  positions 
of  the  moon  be  marked  upon  the  ecliptic  with  chalk,  beginning  at  the 
equinox,  when  the  globe  is  revolved  thesje  marks  will  appear  at  the 
horizon  only  a  short  time  apart. 


THE  MOON. 


133 


is  much  nearer  to  us  than  any  other  JiejiyjmlyjDody, 
and  for  small  telescopes  it  is  therefore  one  of  the  most 
interesting  and  beautiful  objects  in  the  heavens.  The 


FIG.  30. — THK  MOON  NKAB  THE  LAST  QUARTER.    (From  Newcomb's  Popular  Astronomy.) 

best  time  for  evening  ^observation  of  the  moon  is  from 
the  time  it  is  new  until, it  is  half  full.     The  inner  edge 


134 


ASTRONOMY. 


of  the  crescent,  separating  the  light  and  dark  parts  ot 
the  moon,  called  the  terminator,  will  be  very  uneven  and 


tflQ.  31.— A   GROUP  OF   LXJNAK    MOUNTAINS. 


jagged.    This  is  caused  by  the  rough  mountainous  sur- 
face of  the  moon.     Near  the  terminator,  round  pits,  re- 


THE  MOON. 


135 


sembling  pock-marks,  will  probably  be  seen  ;  these  are 
the  great  crater  mountains.  It  is  their  dark  shadows 
beside  them  that  make  all  these  mountains  and  craters 
distinct.  At  full  moon  these  shadows  disappear,  for  the 
sun  is  then  shining  directly  down  upon  them.  The 


FIQ.  32.— THE  LUNAK  MOUNTAIN,  COPERNICUS. 

bright  rays  or  streaks  that  run  out  in  all  directions  from 
some  of  the  mountains  constitute  the  most  interesting 
feature  of  the  full  moon? 

128.   The  Geography  of  the  Moon. — The  dark  patches 
on  the  moon's  surface  were  formerly  supposed  to  b«* 


136  ASTRONOMY. 


seas,  and  were  accordingly  named  as  the  different 
seas  on  the  earth  are  named ;  and,  although  it  is  now 
known  that  there  is  no  water  on  the  moon  (Art.  130), 
they  are  still  called  seas.  They  seem  to  be  great 
plains,  like  our  prairiesrT'he  moon's  surface  is  very 
mountainous.  By  measuring  the  length  of  their  shad- 
ows the  height  of  many  of  these  mountains  has  been 
calculated.  Some  of  them  are  over  25,000  feet  high, 
nearly  or  quite  as  high  as  any  upon  the  earth,  although 
the  moon  is  so  much  smaller  than  the  earth.  The 
larger  mountains  have  been  named  after  eminent  sci- 
entific men. 

129.  The  Crater  Mountains  are  the  most  curious  fea- 
ture of  the  moon.  They  are  saucer-like  depressions 
surrounded  by  ring-mountains,  and  resemble  the  cra- 
ters of  our  volcanoes,  but  many  of  them  are  on  a  much 
larger  scale.  Sharp  peaks  often  rjse  from  the  middle 
of  the  craters,  like  the  cones  in  our  volcanic  craters. 
Fig.  31  shows  a  part  of  the  moon's  surface  covered 
with  such  craters  of  all  sizes,  while  Fig.  32  shows  a 
single  one  of  the  largest  of  these  crater  mountains. 
The  diameter  of  this  crater  is  about  fifty  mijes.  The 
bright  rays  running  out  from  the  mountains_seem  to 
be  great  cracks  which  have  filled  up  with  a  whiter  sort 
of  rock. 

The  moon's  surface  has  been  carefully  observed  by 
astronomers,  and  very  accurate  maps  of  it  have  been 
drawn  and  published.1  "We  undoubtedly  have  a  much 

1  The  best  map  of  the  moon  yet  published  is  by  two  German  astron- 
omers, Beer  and  Maedler.  It  is  two  and  a  half  feet  in  diameter.  Dr. 
Schmidt,  of  Athens,  Greece,  has  made  one  nearly  eight  feet  in  diam- 
eter, which  has  been  published  at  the  expense  of  the  German  Gov- 
ernment. 


THE  MOON.  137 


better  knowledge  of  the  outline  features  of  the  side  of 
the  moon  which  we  can  see  than  of  the  great  part  of 

the  earth's  surface^ Magnificent  photographs l  of  the 

moon  have  also  been  taken,  which  picture  with  won- 
derful beauty  and  accuracy  the  moon's  surface  as  seen 
through  a  telescope.  Fig.  30  is  from  a  photograph  by 
Prof.  Henry  Draper. 

130.  No  Air  or  Water  on  the  Moon. — There  is  no  re- 
liable evidence  of  either  air  or  water  upon  the  moon. 
The  moon  in  its  monthly  journey  around  the  earth 
frequently  passes  between  us  and  a_star.  If  the  moon 
had  an  atmosphere,  the  star's  light  would  be  refracted 
by  it,  just  as  it  goes  behind  the  edge  of  the  moon. 
This  refraction  would  displace  the  star  slightly,  just  as 
refraction  by  our  atmosphere  does  (Art.  105).  But  the 
most  careful  observations  of  these  occultations?  as  they 
are  called,  fail  to  show  any  such  displacement.  This  does 
not  prove  that  the  moon  is  absolutely  without  atmos- 
phere, but  does  prove  that  if  there  is  any  it  must  be 
very  insignificant.  Besides,  if  the  moon  had  an  at- 
mosphere of  any  extent,  we  should  expect  it  to  make 
the  moon's  surface  somewhat  indistinct,  and  also  to 
cause  a  twilight  or  sort  of  shading  along  the  edge  of 
the  bright  part  of  the  moon.  But  nothing  of  the  kind 
can  be  noticed.  The  inequalities  of  the  moon's  sur- 
face stand  out  with  the  utmost  sharpness  and  distinct- 


1  The  best  photographs  of  the  moon  yet  taken  are  by  Mr.  Ruther- 
ford, of  New  York. 

Mr.  Henry  Harrison,  of  New  York,  is  publishing  six  very  accurate 
and  beautiful  colored  representations  of  the  moota  at  different  phases. 
The  diameter  of  the  moon  is  eighteen  inches,  and  all  the  features  arc 
distinctly  given.  The  colors  are  such  as  are  seen  in  the  telescope. 

2  Occulta'tion,  from  Latin  occultatio,  a  hiding. 


138  ASTRONOMY. 


ness,  and  the  terminator,  the  boundary-line  between 
day  and  night,  is  perfectly  defined,  giving  no  evidence 
of  a  twilight.  Observations  with  the  spectroscope 
(p.  298)  also  prove  that  there  is  no  considerable  atmos- 
phere on  the  moon. 

If  there  were  water  upon  the  moon,  it  would  evap- 
orate and  form  an  atmosphere  of  vapor  as  easily  de- 
tected as  one  of  air.  The  visionary  idea  has  been  ad- 
vanced that  the  moon's  centre  of  gravity  is  so  much 
nearer  the  other  side  of  the  moon  that  all  the  air  and 
water  have  run  around  to  the  opposite  side,  which  we 
never  see.  But  there  is  no  evidence  of  anything  of  the 
kind.  In  all  probability,  the  unseen  side  of  the  moon 
is  very  much  like  the  side  we  do  see. 

131.  Is  the  Moon  Inhabited  ? — The  absence  of  air  and 
water  at  once  answers  this  question  in  the  negative. 
Besides,  changes  of  heat  and  cold  upon  the  moon  must 
be  so  extreme  as  to  destroy  any  sort  of  living  beings 
that  we  know  anything  of.     Two  :rceeks  of  constant 
sunshine,  unchecked  by  any  atmosphere  or  the  slight- 
est cloud,  would  be  an  extremely  uncomfortable  blaze, 
not  necessarily,  however,  raising  the  temperature  very 
high,  for  we  know  that  snow  can,  exist  .under  some- 
what  similar   circumstances  on   our  high  mountains. 
During    the    two   weeks   of   night    which   follow   an 
intense  cold   would   prevail,  carrying  down  the  tem- 
perature probably  much  below  the  lowest  records  of 
our  polar  regions.     We   may  therefore   assume   that 
life,  such  as  is  represented  by  the  common  terrestrial 
animals  and  plants,  does  not  exist  on  the  moon. 

132.  The  Moon's  Past  and  Present. — The  moon  gives 
strong  evidence  of  having  been  at  one  time  molten. 
The  great  craters  were  probably  formed  then  by  the 


THE  MOON.  139 


bursting  forth  of  great  bubbles  of  gas  from  the  inte- 
rior. There  is  also  clear  evidence^  of  volcanic  action 
there  in  the  past.  But  there  is  no  trustworthy  evidence 
that  any  such  volcanic  action  has  ever  been  seen  by 
men.  The  moon's  volcanoes  have  probably  been  long 
extinct.  There  has,  indeed,  been  considerable  evidence 
of  change  in  the  appearance  of  some  small  portions  of 
the  moon's  surface  in  late  years,  but  the  matter  is  still 
in  doubt.  In  order  to  account  for  the  absence  of  air 
and  water  upon  the  moon,  it  has  been  suggested  that, 
as  the  moon  cooled  off,  the  contraction  in  its  interior 
caused  great  caverns  there  ,^  which  have  swallowed  up 
its  atmosphere  and  oceans.  The  idea  is  ingenious  and 
not  improbable,  but  of  course  nothing  is  really  known 
about  it.  Whether  the  moon  ever  contained  life  we 
cannot  say,  but  it  is  now  a  dead,  sterile  mass  of  rock. 

133.  The  Occultation  of  Stars  by  the  Moon. — An  oc- 
cultation  of  a  star  by  the  moon  is  an  interesting  and 
at  first  a  surprising  sight.  Small  stars  are  frequently 
occulted,  the  larger  ones  and  the  planets  more  rarely. 
The  moon's  brightness  overpowers  the  light  of  all  but  the 
brightest  stars  as  they  come  to  its  edge,  so  that  a  tele- 
scope is  needed  to  watch  their  occultation.  If  the  star 
disappears  at  the  dark  edge  of  the  moon,  which  is  always 
the  case  if  the  occultation  comes  before  full  moon,  the 
phenomenon  is  very  striking.  The  star  disappears  sud- 
denly and  apparently  without  there  being  anything  to 
ca,use  its  disappearance.  Its  reappearance  on  the  other 
side  is  just  as  sudden.  This  proves  that  the  stars  have 
no  apparent  size  whatever,  but  are  mere  points  of  light 
(Art.  204),  and  also  furnishes  a  further  proof  of  the  ab- 
sence of  atmosphere  or  vapor  on  the  moon,  which  would 
make  it  fainter  just  before  its  disappearance.  An  occul- 


140 


ASTRONOMY. 


tation  of  one  of  the  prominent  planets  is  rather  rare, 
and  always  attracts  attention.  As  in  the  telescope  these 
show  disks  of  considerable  size,  they  disappear  gradu- 
ally. Fig.  33  shows  an  occupation  of  Jupiter.  All  the 
occultations  of  stars  bright  enough  to  be  seen  by  the 

naked  eye  are  predicted 
every  year  in  the  Nau- 
tical Almanac,1  for  the 
use  of  observers.  Stu- 
dents having  the  use  of 
telescopes  should  be  on 
the  lookout  for  them. 

134.  Light  and  Heat 
from  the  Moon. — Several 
astronomers  have  tried  to 
determine  the  amount  of 
light  shed  by  the  moon. 
The  results  are  generally  expressed  by  comparing  the 
moon's  light  with  that  of  the  sun.  The  latest  and  most 
reliable  result  is,  that  the  sun  gives  600,000  times  as 
much  light  as  the  full  moon.  Chambers  aptly  says 
that  "  if  the  whole  sky  were  covered  with  full  moons 
they  would  scarcely  make  daylight." 

Until  recently,  not  the  slightest  heat  from  the  moon 
could  be  detected.  But  by  concentrating  its  heat  with 
the  largest  telescopes,  and  using  the  most  delicate  ap- 
paratus, it  has  finally  been  detected  and  measured.  It  is 
found  that  the  moon's  heat  would  raise  the  mercury  in  a 
thermometer  only  about  5^^  of  a  degree  Fahrenheit.1 


FIG.  33. — AN  OCCCLTATION  OF  JUPITER. 


1  See  foot-note  on  page  169. 

3  So  slight  a  rise  in  temperature  could  not  be  noticed  in  any  ther- 
mometer, even  if  the  moon's  heat  were  concentrated  upon  it  by  the 
largest  telescope  in  the  world.  The  Thermo-electric  Pile,  by  which 


THE  MOON.  141 


So  that,  while  the  moon's  light  is  a  great  advantage  to 
us,  us  heat  does  nothing  to  warm  us. 

135.  The  Moon  and  the  Weather. — It  is  very  com- 
monly believed  that  the  moon  exercises  great  influence 
upon  the  weather.  This  is  a  mere  superstition.  No 
good  reason  for  this  supposed  influence  has  ever  been 
given,  and  accurate  records  of  the  weather  kept  for 
many  years  show  that  not  the  least  reliance  can  be 
placed  upon  the  moon's  "  weather  signs."  It  is  true 
that  various  scientific  men  have  attempted  to  prove 
that  there  is  on  the  average  a  slight  difference  in  the 
rainfall  at  different  phases  of  the  moon.  But,  unfor- 
tunately, they  seem  to  come  to  directly  opposite  about 
as  often  as  to  the  same  conclusions ;  and  the  greatest 
difference  thus  claimed  by  any  of  them  is  so  slight  that 
ordinary  observation  would  never  notice  it  at  all.  So 
that  we  may  fairly  conclude  that  any  such  differences  in 
the  amount  of  rainfall  at  different  ages  of  the  moon  are 
simply  accidental,  and  will,  as  likely  as  not,  be  reversed 
during  the  next  period  of  equal  length.1 

The  best-supported  of  these  theories  is  that  the  heat 
of  the  full  moon  does  something  to  clear  away  clouds. 
If  the  moon  has  any  such  effect, — which  is  doubtful, — 
it  must  be  inconsiderable.  The  notion  that  the  signs 
of  the  moon  in  which  they  were  planted  influence  the 

the  slightest  heat  starts  a  current  of  electricity,  was  used  in  the  ex- 
periments. 

1  The  conspicuous  failures  of  many  reputed  weather  prophets  should 
convince  people  of  the  absolute  impossibility  in  the  present  state  of 
science  of  foretelling  the  weather  for  any  particular  day  more  than  a 
day  or  two  in  advance. 

It  is  scarcely  necessary  to  remark  that  the  weather  prognostications 
so  often  to  be  found  in  our  common  almanacs  have  not  the  slightest 
Talue. 


142  ASTRONOMY. 


growing  of  the  crops  is  still  extremely  prevalent,  es- 
pecially among  farmers.  This  is  still  more  absurd  than 
the  belief  in  the  moon's  influence  upon  the  weather. 
There  is  nothing  in  reason  or  in  facts  to  warrant  any 
such  beliefs.  They  are  as  foolish,  if  not  so  hurtful,  as 
was  our  forefathers'  belief  in  witches. 

136.  Appearance  of  the  Eo.rth  from  the  Moon. — From 
the  moon  the  earth  would  seem  to  be  a  splendid  large 
moon,  having  nearly  four  times  the  diameter  and  thir- 
teen times  the  surface  of  our  moon.  The  earth  would 
go  through  its  phases  just  as  the  moon  does  for  us,  but 
these  phases  would  be  exactly  opposite  to  those  of  the 
moon.  When  the  moon  is  new  the  earth  would  loefull, 
and  while  the  moon  increases  from  new  to  full  the 
earth  would  decrease  from  full  to  new. 

Every  one  has  noticed  that  when  the  moon  is  only  a 
few  days  old  the  dark  part  is  faintly  lighted  up,  and 
the  whole  moon  can  be  distinctly  seen.  This  is  often 
called  "  the  old  moon  in  the  new  moon's  arms."  It  is 
simply  the  earth-shine  lighting  up  the  moon's  surface, 
or,  as  it  might  fairly  be  called,  moonlight  on  the  moon. 
It  can  be  seen  as  well  before  as  after  new  moon.1 

Since  the  same  side  of  the  moon  is  always  turned 
towards  us,  the  earth  to  an  observer  on  the  moon  would 
never  rise  or  set.  At  any  one  place  on  the  moon  it 
would  always  be  seen  in  the  same  place  in  the  sky,2 

1  Why  should  it  be  seen  about  the  time  of  new  moon  ? 

2  Although  the  earth  would  stay  at  about  the  same  place  in  the  sky, 
it  would  not  stay  at  the  same  place  among  the  stars.     They  would 
rise  and  set  as  they  do  for  us,  but  would  take  two  weeks  instead  of 
twelve  hours  to  pass  from  east  over  to  west. 

.Because  there  is  no  atmosphere  to  reflect  the  sun's  rays  everywhere, 
and  thus  overpower  their  light,  the  stars  could  be  seen  upon  the  moon 
day  and  night,  and  the  sky  itself  would  always  be  intensely  black. 


THE  MOON.  J43 


subject  only  to  slight  oscillations  from  the  librations. 
And  there  it  would  go  through  all  its  phases,  growing 
from  new  to  full  and  waning  back  to  new  again.  To 
an  observer  about  the  middle  of  our  side  of  the  moon 
the  earth  would  be  always  overhead.  To  one  near  the 
edge  it  would  always  be  on  the  horizon,  while  on  the 
opposite  side  it  would  never  be  seen. 

Notwithstanding  its  size,  it  is  not  likely  that  the  fea- 
tures of  the  earth's  surface  could  be  distinctly  seen  from 
the  moon.  Our  atmosphere,  by  reflecting  the  sun's 
light,  and  by  obstructing  the  light  coming  through  it 
from  the  earth,  would  always  produce  an  indistinct- 
ness, and  clouds  would  of  course  completely  hide  every- 
thing beneath  them.  It  is  probable  that  the  earth  is 
a  better  reflector  of  light  than  the  moon,  and  would 
therefore  be  more  brilliant  as  well  as  larger  than  the 
moon. 


144  ASTRONOMY. 


CHAPTER    VI. 

ECLIPSED 

137.  Causes  of  Eclipses.— The  moon  is  eclipsed  by 
passing  into  the  earth's  shadow.     As  the  moon  shines 
by  reflecting   sunlight,  and   the  earth  then  cuts  off 
this  sunlight,  the  moon  is  of  course  darkened.     When 
this  occurs,  the  moon  must  be  on  the  opposite  side  of 
the  earth  from  the  sun :  so  that  an  eclipse  of  the  moon 
can  occur  only  at  full  moon.  |  The  sun  is  eclipsed  when 
the  moon  passes  directly  between  the  sun  and  the  earth. 
The  sun  and  the  moon  must  then  be  on  the  same  side 
of  the  earth.    An  eclipse  of  the_sun,  therefore,  can  occur 
only  at  new  moon. 

138.  Why  Eclipses  do  not  occur  at  every  New  and  Full 
Moon. — If  the  moon's  orbit  were  in  the  plane  of  the 
earth's  orbit,  as  it  seems  to  be  in  Fig.  34,  we_should 
have  an  jecjipse  of^th^sun  at  every  new  moon,  for  it 
wjould  always  pass  then  between  the  earth  and  the  sun, 
and  at  evej^Lfidl  nioon  we  should  have  an  eclipse  of  the 
moon.     But  the  moon's  orbit  is  inclined  to  the  earth's 
orbit  at  a  small  angle  (about  5°).     It  is  just  as  if  one 
were  to  take  hold  of  the  moon's  orbit  at  0  (Fig.  34) 
and  lift  that  side  of  it  up  a  little  way,  letting  it  swing 
about  an  axis  from  A  to  B.     The  side  T  would  be  as 
far  below  the  page  as  O  is  above  it.     The  page  on 
which  the  rest  of  the  figure  still  lies  represents  the 
plane  of  the  ecliptic  or  earth's  orbit.     The  points  A 


ECLIPSES.  145 


and  B  would  be  the  moon's  nodes  (Art.  32).  Because 
of  this  Jnclination  of  its  orbit^jthe  moon  in  passing 
around  the  earth  generally  passes  a  little  way  above  or 
below  the  sun  and  the  earth's  shadow,  and  so  there 
is^  up  eclipse. 

139.  The  Eclipse  Seasons. — "When  the  earth  is  on  or 
near  the  line  along  which  the  two  orbits  intersect,  the 
axis  AB  about  which  we  turned  the  moon's  orbit  in 
the  last  article,  then  one  or  more  eclipses  will  occur. 
The  earth  in  passing  around  its  orbit  must  cross  this 
line   twice   a  year.1     The   eclipses,  therefore,  are   all 
grouped  together  in  two  seasons,  nearly  six  months 
apart.     They  can  only  accur  within  seventeen  or  eigh- 
teen days  of  the  date  when  the  earth  crosses  this  line, 
called  the  line  of  the  moon's  nodes.     The  eclipse  sea- 
sons do  not  occur  at  the  same  dates  each  year,  but 
move  forward  about  nineteen  days.     That  is,  if  eclipses 
take  place  on  or  near  January  1st,  the  next  time  will 
be  about  June   21st,  and  the  next  about  December 
12th. 

140.  Shadows  cast  by  the  Moon  and  the  Earth. — Be- 
cause the  sun  is  larger  than  the  earth  or  the  moon,  the 
shadows  cast  by  these  bodies  are  cones :  they  taper  off 


1  A  little  thought  will  make  this  clear.  The  earth  in  Fig.  34 
moves  around  the  sun  in  a  direction  opposite  to  the  motion  of  the 
hands  of  a  watch,  carrying  the  revolving  moon  along  with  it.  The 
line  between  the  earth  and  the  sun  will  at  first  run  above  the  Tside 
of  the  moon's  orbit.  As  we  look  at  the  figure,  the  new  moon  now 
passes  above  the  line  between  the  earth  and  the  sun,  and  the  full  moon 
under  the  shadow,  causing  no  eclipses.  In  about  six  months  the  earth 
is  at  the  line  of  nodes  again,  and  the  eclipses  occur.  For  the  next 
six  months  the  line  between  the  sun  and  the  earth  is  below  the  O  side 
of  the  moon's  orbit,  and  the  new  moon  passes  under  and  the  full 
moon  above,  causing  no  eclipses  as  before, 

13 


146  ASTRONOMY. 


to  a  point.  The  earth's  shadow  is  of  course  larger  and 
longer  than  the  moon's.  Fig.  34  shows  parts  of  these 
shadows.  To  an  observer  in  either  of  these  shadows 
the  sun  is  entirely  hidden. 

If  one  were  between  the  earth's  shadow  and  the  line 
MC  or  ND,  it  is  evident  that  the  earth  would  hide  a 
part  of  the  sun.  This  space  around  the  shadow  is 
called  the  penumbra.  The  nearer  one  is  to  the  edge  of 
the  shadow  the  greater  is  the  part  of  the  sun  hidden, 
and  the  less  light  there  is.  The  penumbra  does  not 
grow  narrower  and  finally  come  to  an  end,  like  the 
shadow,  but  grows  wider  constantly.  The  moon's 
penumbra  is  also  shown  in  the  figure. 

141.  Eclipses  of  the  Moon. — When  the  moon  passes 
entirely  into  the  earth's  shadow,  the  eclipse  is  total. 
When  only  one  side  of  the  moon  passes  through  the 
shadow,  the  eclipse  is  partial.  Although  the  almanac 
gives  the  time  when  the  moon  enters  the  penumbra,  yet 
the  light  shed  upon  it  then  is  so  little  diminished  that 
it  cannot  be  noticed.  There  is  no  perceptible  dimming 
of  the  moon  until  it  almost  reaches  the  shadow.  When 
the  moon  enters  the  shadow,  a  notch  seems  to  be  cut  out 
of  the  moon.  This  notcn  is  always  round,  proving,  as 
mentioned  in  Art.  63,  that  the  shadow  must  be  round, 
and  therefore  that  the  earth,  which  casts  it,  must 
be  a  sphere.  In  a  partial  eclipse  this  round  notch 
growls  larger  and  larger  until  the  middle  of  the  eclipse, 
then  it  grows  less  until  it  is  over.  In  a  total  eclipse 
the  shadow  gradually  covers  the  whole  moon. ,  When 
totally  eclipsed,  the  moon  can  generally  still  be  seen, 
shining  with  a  faint  jreddish  light.  This  is  caused  by 
the  sun's  rays  being  bent  around  the  earth  to  the  moon 
by  the  refraction  of  the  earth's  atmosphere  (Art.  105). 


ECLIPSES.  147 


The  light  which  thus  reaches  the  moon  is  red,  because 
the  moisture  in  the  earth's  atmosphere  absorbs  the 
other  colors  of  the  sunlight,  but  allows  the  red  to  pass 
through.  The  sun  itself  sometimes  looks  red  when 
rising  or  setting,  from  the  same  cause.  "When  only  a 
small  part  of  the  moon  is  eclipsed,  that  part  is  gener- 
ally entirely  invisible  to  the  eye,  because  the  brightness 
of  the  rest  overpowers  its  feeble  light. 

The  magnitude  of  an  eclipse  is  expressed  by  men- 
tioning the  fractional  part  of  the  moon's  diameter 
which  is  covered  by  the  shadow.  If  the  magnitude  is 
1,  the  eclipse  is  just  total ;  if  more  than  1,  the  moon  is 
farther  within  the  shadow.  The  greatest  possible  du- 
ration of  a  total  eclipse  of  the  moon  is  about  one  and 
three-fourths  hours. 

142.  Eclipses  of  the  Sun. — When  the  moon  passes  be- 
tween the  sun  and  the  earth,  the  sun  is.  eclipsed.1  At 
all  places  in  the  moon's  shadow  (see  Fig.  34)  the  sun 
is  wholly  hidden,  and  the  eclipse  is  total.  In  the  pe- 
numbra the  sun  is  partly  hidden,  and  the  eclipse  is 
partial.  The  sun  and  the  moon  are  apparently  about 
the  same  size ;  but,  as  the  distances  of  both  from  the 
earth  vary  somewhat  (Arts.  70, 120),  their  apparent  sizes 
vary  a  little.  When  the  moon  is  nearest  to  us,  and  the 
sun  farthest  off,  the  moon  will  seem  larger  than  the 
aun,  and  will  entirely  cover  it.  The  moon's  shadow 
then  reaches  the  earth,  as  shown  in  the  figure,  and  the 
eclipse  is  total.  But  if  at  the  time  of  the  eclipse  the 


1  It  may  not  be  amiss  to  remark  that  the  darkness  which  followed 
the  crucifixion  of  Christ  could  not  have  been  caused  by  a  solar 
eclipse,  for  the  feast  of  the  Passover,  during  which  the  crucifixion 
took  place,  was  always  held  &tfull  moon. 


148 


ASTRONOMY. 


Fio.  34. — ECLIPSES  OF  THE  SUN  AND  MOON. 


sun  is  nearest  to  us, 
and  the  moon  farthest 
off,  the  moon  would 
seem  smaller  than  the 
sun,  and  would  not 
hide  it  all.  The 
moon's  shadow  would 
not  reach  quite  to  the 
earth.  In  this  case  a 
ring  of  light  would 
be  seen  around  the 
edge  of  the  sun.  The 
eclipse  is  not  total, 
but  annular.1 

143.  Total  Eclipses 
of  the  Sun. — The  sec- 
tion of  the  moon's 
shadow  which  falls 
upon  the  earth  is  very 
narrow,  never  as  much 
as  200  miles  wide. 
Arid  though  two  or 
more  solar  eclipses 
occur  every  year,  yet 
a  total  eclipse  of  the 
sun  is  a  very  rare  oc- 
currence at  any  one 
place.  The  next  one 
visible  in  the  eastern 
United  States  will  be 
in  the  year  1900.  Par- 


1  An/nular,  from  Latin  annulus,  a  ring. 


ECLIPSES. 


149 


Fio.  35.— A  TOTAL  ECLIPSE  or  THE  SUN. 


tial  eclipses  of  the  sun  may  last  two  or  three  hours,  but 
total  eclipses  rarely  exceed  five  or  six  minutes.  Yet  so 
wonderful  is  the  sight,  and  so  great  are  the  opportuni- 

13* 


150  ASTRONOMY. 


ties  then  afforded  of  studying  the  sun,  that  astronomers 
travel  thousands  of  miles  to  witness  them.  A  total 
eclipse  of  the  sun  is  one  of  the  sublimest  of  phenom- 
ena. The  crescent  of  sunlight  becomes  narrower  and 
narrower,  until  presently  the  great  shadow  is  seen 
rushing  over  the  earth  towards  us  with  immense  rapid- 
ity, and  in  an  instant  is  upon  us.  The  surface  of  the 
moon  is  as  black  as  ink.  At  various  places  on  its  edge 
the  red  prominences  stand  out  in  fantastic  shapes,  great 
tongues  of  flame  projecting  many  thousands  of  miles 
beyond  the  sun's  chromosphere,  from  which  they  come. 
Surrounding  this  is  the  silvery  corona,  brilliant  at  the 
sun's  edge,  but  fading  out  to  imperceptibility.  The 
darkness  varies  according  to  the  duration  of  the  eclipse 
and  the  clearness  of  the  sky,  but  it  is  usually  too  great 
to  allow  ordinary  print  to  be  read.  The  brighter  stars 
are  visible.  Animals  seem  to  think  that  night  has  come. 
No  wonder  that  uncivilized  peoples  have  always  feared 
eclipses  of  the  sun.  We  cannot  behold  them  without 
awe.  Suddenly  the  light  bursts  forth,  the  great  shadow 
flies  away  as  fast  as  it  came,  and  the  most  wonderful 
spectacle  of  the  generation  is  over. 

144.  Number  of  Eclipses. — At  every  eclipse  season 
there  is  certain  to  be  one  eclipse  of  the  sun,  and  there 
may  be  two.  Besides,  the  moving  of  the  eclipse  season 
backward  nineteen  days  a  year  may  throw  a  part  of  a 
third  eclipse  season  into  the  year.  The  least  number  of 
solar  eclipses  that  can  occur  in  a  year  is  two,  and  the 
greatest  number  is  five. 

The  moon  can  be  eclipsed  only  once  at  each  eclipse 
season,  and  may  not  be  eclipsed  at  all.  The  num- 
ber of  lunar  eclipses  in  a  year  varies  from  none  to 
three.  The  greatest  possible  number  of  eclipses  in  a 


ECLIPSES.  151 


year  is  seven,  of  which  four  or  five  are  of  the  sun,  and 
two  or  three  of  the  moon.  The  least  possible  number 
is  two,  both  of  the  sun.  "We  may  see  from  Fig.  34  why 
there  are  more  eclipses  of  the  sun  than  of  the  moon. 
The  sun  is  eclipsed  whenever  the  moon  is  partly  or 
wholly  between  G  and  H,  but,  because  the  shadow 
tapers,  the  distance  HG  is  greater  than  the  width  of 
the  shadow  at  B ;  the  moon  will  therefore  oftener  pass 
between  the  earth  and  the  sun  than  it  will  pass  through 
the  earth's  shadow. 

145.  Why  we  see  more  Eclipses  of  the  Moon  than  of  the 
Sun. — Although  eclipses  of  the  sun  are  about  one  and 
one-half  times  as  numerous  as  those  of  the  moon,  yet 
at  any  one  place  on  the  earth  lunar  eclipses  are  more 
frequently  seen.    This  is  because  the  moon's  shadow  in 
a  solar  eclipse  does  not  cover  the  whole  earth,  and  the 
eclipse  is  seen  only  at  those  places  which  happen  to  be 
in  the  path  of  the  shadow  or  penumbra.     But  in  a 
lunar  eclipse  the  moon's  light  is  put  out,  and  the  eclipse 
is  seen  over  all  that  half  of  the  earth  which  is  then 
turned  towards  the  moon. 

146.  Calculation  and  Prediction  of  Eclipses —  The  Saros. 
— When  and  where  eclipses  will  be  seen  can  be  calcu- 
lated beforehand  with  great  accuracy ;  but  considerable 
knowledge  of  mathematics  is  required  for  this.     They 
are  always  announced  in  our  almanacs,  and  are  fre- 
quently worked  out  many  years  in  advance. 

Long  before  eclipses  could  be  calculated,  men  found 
by  observation  that  they  repeated  themselves  so  exactly 
about  every  eighteen  years1  that  they  could  by  this 

1  More  exactly,  eighteen  years  and  ten  or  eleven  days,  according  as 
four  or  five  leap-years  are  included. 


152  ASTRONOMY. 


means  be  predicted.  This  is  because  at  the  end  of  this 
time  the  sun,  moon,  and  earth  are  in  almost  precisely 
the  same  positions  as  at  the  beginning,  so  that  they 
again  describe  the  same  paths,  and,  with  only  an  occa- 
sional exception,  cause  the  same  eclipses,  as  in  the  past 
eighteen  years.  This  round  of  eclipses  was  called  by 
the  ancients  the  Saros.  The  discovery  of  the  Saros  is 
credited  to  the  Chaldeans,1  who  predicted  eclipses  by 
it  hundreds  of  years  before  Christ. 

147.  Eclipses  as  seen  from  the  Moon. — When  we  have 
an  eclipse  of  the  sun,  an  observer  on  the  moon  would 
see  only  a  small,  round,  ill-defined  shadow  crossing  the 
earth.  It  would  be  a  partial  eclipse  of  the  earth  (Fig. 
34).  The  earth  could  never  be  totally  eclipsed. 

When  we  have  a  total  eclipse  of  the  moon,  the  sight 
from  the  moon  would  be  a  very  strange  one.  The  sun 
would  be  wholly  behind  the  earth,  and  hence  totally 
eclipsed,  but  his  light  would  be  so  refracted  by  the 
earth's  atmosphere  that  a  dull  red  ring  of  light  would 
surround  the  black  earth. 

1  Eclipses,  especially  total  eclipses  of  the  sun,  were  greatly  dreaded 
by  the  ancients,  and  are  still  dreaded  by  uncivilized  peoples.  The 
Hindoos  believe  that  in  a  solar  eclipse  some  monster  is  trying  to 
swallow  the  sun.  At  these  times  they  all  turn  out  with  gongs  and 
every  possible  noise-producing  instrument,  and  keep  up  the  loudest 
and  most  hideous  noises  until  the  frightened  monster  disgorges  his 
fiery  mouthful. 

The  foreknowledge  of  an  eclipse  of  the  moon  was  once  of  great 
service  to  Columbus,  when,  in  1504,  he  was  wrecked  off  the  coast  pf 
Jamaica.  When  neither  threats  nor  persuasion  would  induce  the 
natives  to  furnish  him  with  food,  he  told  them  that  their  Great  Spirit 
was  displeased  with  them  for  their  treatment  of  him,  and  that  the 
moon  would  that  night  be  darkened.  When  the  eclipse  came,  the 
Indians  were  convinced  that  Columbus  had  told  the  truth,  and 
hastened  to  bring  supplies  for  himself  and  his  crew,  beseeching  him 
to  pray  that  the  Great  Spirit  might  receive  them  again  into  his  favor. 


THE  SUPERIOR  PLANETS.  153 


CHAPTER  VII. 

THE   SUPERIOR   PLANETS. 
MAES.     <f 

Distance^from  the  Sun,  142,000,000  Miles.  Diameter,  4200 
Miles.  Length  of  Year,  2  Years.1  Length  of  Day,  241  Hours. 
Specific  Gravity,  4.  Two  Satellites. 

148.  Relations  to  the  Solar  System. — Next  outside  of 
the  earth's  orbit  comes  Mars,2  the  last  of  the  group  of 
four  smaller  planets  to  which  the  earth  belongs.     Ex- 
cept Venus,  Mars  is  the  nearest  of  all  the  planets  to 
us ;  and  next  to  Mercury,  it  is  the  smallest  of  the  prin- 
cipal planets.     Its  surface  is  but  little  more  than  one- 
fourth,  and  its  mass  is  about  one-ninth,  of  those  of  the 
earth.     One  would  there  receive  less  than  one-half  the 
light  and  heat  that  he  receives  upon  the  earth. 

149.  Motions    and    Phases. — Unlike    Mercury    and 
Venus,  Mars  in  its  revolution  around  the  sun  passes 
entirely  around  the  earth.     Its  motion  among  the  stars 
is  generally  direct,  or  eastward,  but  when  near  opposi- 
tion, as  explained  in  Art.  24,  it  seems  to  us  to  move 
westward.    Being  an  outside  planet,  Mars  is  never  seen 
as  a  crescent,  but  when  90°  from  the  sun  the  telescope 

1  These  tabular  statements  are  given  in  round  numbers  for  con- 
venience in  remembering  them.     For  the  exact  data,  see  Art.  27. 

2  Mars  was  the  god  of  war.     The  symbol  of  the  planet  (cf)  repre- 
sents his  shield  and  spear. 


164  ASTRONOMY. 


shows  it  to  be  decidedly  gibbous.  It  is  then  of  about 
the  same  shape  as  the  moon  three  days  before  or  after 
being  full.1  At  opposition  Mars  is  nearest  to  the  earth, 
and  of  course  brightest.  The  oppositions  occur  every 
twenty-five  or  twenty-six  months.  The  average  dis- 
tance of  Mars  from  the  earth  at  opposition  is  less  than 
fifty  millions  of  miles,  but  if  the  opposition  should 
occur  when  Mars  is  nearest  to  the  sun  and  the  earth  is 
farthest  from  the  sun,  their  distance  apart  would  be 
only  thirty-five  millions  of  miles. 

150.  Description  of  Mars. — To  the  naked  eye  the 
most  noticeable  feature  about  Mars  is  his  fiery-red 
color:  he  is  the  reddest  of  all  the  heavenly  bodies. 
Although  he  does  not  come  so  near  to  us  as  Venus, 
yet  Mars  is  in  a  better  position  for  observation  than 
any  of  the  rest  of  the  planets  except  our  own,  because 
at  opposition,  when  he  is  nearest  to  us,  the  whole  of 
his  bright  surface  is  turned  towards  us,  while  when 
Venus  is  nearest  to  us  her  dark  side  is  towards  us. 
The  moon  alone,  of  all  the  heavenly  bodies,  is  bettei 
situated  in  this  respect.  Indeed,  we  are  not  certain 
that  the  real  surface  of  any  of  the  rest  of  the  planets 
has  ever  been  seen.  In  a  large  telescope  patches  of  dif- 
ferent colors  are  seen  upon  the  surface  of  Mars  which 
bear  a  strong  resemblance  to  land  and  water.  The 
parts  supposed  to  be  seas  have  a  greenish  hue,  like  our 
seas,  while  the  parts  which  have  been  taken  to  be  land 
are  red,  which  has  been  ascribed  to  the  color  of  the 
soil  and  rocks,  perhaps  like  our  red  sandstone.  The 
red  parts  overpower  the  green,  and  give  their  color  to 
the  planet.  Maps  and  globes  of  Mars  have  been  made, 

1  Can  there  be  a  transit  of  Mars  ? 


THE  SUPERIOR  PLANETS. 


155 


upon  which  these  supposed  continents  and  seas  are 
drawn  and  named,  the  names  given  them  being  those 
of  various  famous  astronomers.  Unlike  the  earth, 
Mars  seems  to  have  more  land  than  water,  and  the 
seas  there  are  long  and  narrow.  But  the  most  striking 
features  of  Mars's  surface  are  two  brilliant  white  spots 
near  his  poles.  They  are  probably  ice  and  snow,  such 
as  are  found  about  the  poles  of  the  earth.  And  they 
seem  to  decrease  when  in  summer  they  are  turned 
towards  the  sun,  and  to  increase  again  when  turned 
from  the  sun  in 
winter,  just  as  the 
ice  and  snow 
about  the  earth's 
poles  do.  Be- 
sides these  resem- 
blances to  the 
earth,  Mars  has 
seasons  like  ours. 
His  equator  makes 
a  somewhat  larger 
angle  with  his  or- 
bit than  the  angle 
between  our  equa- 
tor and  the  eclip- 
tic. And  his  orbit 

is  more  eccentric  than  the  earth's,  so  that  the  ditier- 
ence  between  his  summer  and  winter  distances  from  the 
sun  is  greater.  These  two  causes  make  the  changes 
of  seasons,  then,  greater  than  upon  the  earth.  The 
markings  upon  the  surface  of  Mars  have  enabled  us  to 
determine  the  time  of  his  rotation  with  great  exactness. 
It  is  a  little  more  than  twenty-four  and  one-half  hours. 


FIG.  36.— A  TELESCOPIC  VIEW  OF  MARS. 


156  ASTRONOMY. 


151.  The  Satellites  of  Mars.— Before  1877,  Mars  was 
not  known  to  have  any  satellites.  At  the  opposition 
which  occurred  that  year  the  planet  came  very  near  the 
earth,  and  gave  an  unusually  good  opportunity  of  ob- 
serving him.  At  that  time  Prof.  Hall1  began  to  search 
carefully  for  satellites  of  Mars  with  the  great  twenty- 
six-inch  telescope  in  the  Naval  Observatory  at  Wash- 
ington. In  August  of  that  year  he  found  two  satellites. 
These  satellites  are  very  small  and  very  close  to  the 
planet.  They  are  too  small  to  be  measured,  but,  esti- 
mating their  size  from  the  amount  of  light  they  reflect, 
Prof.  Pickering2  concludes  that  the  diameter  of  the 
inner  is  about  seven,  and  of  the  outer  one  about  six,  miles. 
The  outer  one  is  about  12,000  miles  from  the  surface  of 
Mars,  while  the  inner  one  is  not  quite  4000.  But  the 
most  remarkable  fact  about  the  satellites  is  that  the 
inner  one  revolves  about  Mars  in  about  seven  and  one- 
half  hours,  less  than  one-third  of  the  time  that  it  takes  Mars 
to  turn  on  his  axis.  This  causes  the  inner  satellite  to  rise 
in  the  west  and  set  in  the  east?  To  an  observer  on  Mars 
it  would  seem  to  revolve  around  his  planet  from  west 
to  east  twice  every  day*  all  the  while  going  through  its 
changes  from  new  to  full  and  back  to  new  again  every 

1  Prof.  Asaph  Hall,  attached  to  the  United  States  navy,  and  in 
charge  of  the  great  twenty-six-inch  telescope  at  Washington. 

2  Prof.  E.  C.  Pickering,  Director  of  the  Harvard  College  Observa- 
tory, Cambridge,  Massachusetts. 

3  Since  the  inner  satellite  revolves  around  Mars  from  west  to  east 
faster  than  the  planet  itself  turns  in  that  direction,  it  must  rise  in 
the  west  and  set  in  the  east. 

4  Since  the  planet  turns  once  in  its  daily  motion  while  the  satellite 
revolves  about  it  three  times,  and  both  in  the  same  direction,  the 
satellite  would  seem  to  an  observer  on  Mars  to  revolve  about  his 
planet  only  twice  a  day. 


THE  SUPERIOR  PLANETS.  157 

seven  or  eight  hours.  As  the  outer  moon  rises  in  the 
east  and  sets  in  the  west,  like  ours,  the  two  moons  meet 
each  other  in  the  sky.  The  discovery  of  the  moons  of 
Mars  is  justly  regarded  as  one  of  the  greatest  in  recent 
astronomy. 

152.  Observation  of  Mars. — Although  so  interesting 
an  object  to  the  possessor  of  a  large  telescope,  for  the 
owners  of  small  telescopes  Mars  has  little  interest. 
Small  instruments  show  none  of  the  markings  on  the 
planet  distinctly,  if  at  all ;  and  he  has  very  little  change 
of  phase  such  as  makes  Mercury  and  Venus  interesting. 
As  has  been  said,  Mars  is  much  the  brightest  at  oppo- 
sition, when  his  brilliancy  and  redness  make  him  an 
interesting  object  to  the  naked  eye ;  nor  will  a  small 
telescope  add  much,  if  any,  interest  to  him.  Like  all 
heavenly  bodies  when  in  opposition,  Mars  will  then 
rise  about  the  time  the  sun  sets,  and  shine  all  night. 
This  fact,  together  with  his  fiery  redness,  will  make  it 
easy  to  distinguish  him.  Every  fifteen  or  seventeen 
years  Mars  comes  especially  near  to  the  earth  at  oppo- 
sition. His  brilliancy  then  almost  rivals  that  of  Venus 
and  Jupiter.  These  near  approaches  occur  in  1892  and 
1909.  The  satellites  of  Mars  can  be  seen  only  in  first- 
class  telescopes,1  and  then  for  but  a  few  months  about 


1  The  foolish  statement  that  Mars;s  moons  can  be  seen  by  looking 
at  the  planet  in  a  common  looking-glass  is  sometimes  heard,  and  even 
seen  in  the  newspapers.  The  points  of  light  which  are  seen  in  the 
looking-glass  beside  the  image  of  Mars  are  faint  reflections  of  the 
planet's  light  from  the  inner  and  outer  surfaces  of  the  glass,  while  the 
main  reflection  is  from  the  quicksilver  behind  the  glass.  Any  of  the 
bright  stars,  and  even  the  moon,  will  show  such  "  moons  "  in  a  look- 
ing-glass. But  it  a  piece  of  polished  metal  be  used  as  a  mirror,  they 
will  disappear  from  Mars  as  well  as  from  the  rest. 

14 


158 


ASTRONOMY. 


the  time  of  the  planet's  opposition.  No  telescope  in 
the  world  will  show  them  when  Mars  is  in  the  farther 
part  of  his  orbit. 

Schiaparelli  of  Milan  thinks  he  has  discovered  some 
curious  markings  on  Mars  which  he  calls  canals.  These 
always  end  in  a  sea  or  another  canal,  and  are  perfectly 


FIG.  36.— MAP  OF  MARS. 

straight.  They  are  sometimes  fifty  miles  wide  and  one 
thousand  long,  and  their  exact  nature  is  a  matter  of 
conjecture.  Sometimes  they  seem  to  be  double,  a 
second  line  parallel  to  the  first  being  noticeable. 
They  make  a  network  over  the  continents,  and  some- 
times seem  to  disappear.  The  map  is  a  copy  of 
Schiaparelli's. 


THE  MINOR  PLANETS.          159 


THE  MINOK  PLANETS. 

Distance  from  the  Sun,  200,000,000  to  325,000,000  Miles. 
Diameters,  from  about  300  Miles  down.  Lengths  of  Years, 
3  to  7  Years.  Lengths  of  Days  and  Specific  Gravities  un- 
known. Number  discovered  about  toO. 

153.  Relations  to  the  Solar  System. — Between  the  inner 
group  of  four  small  planets  and  the  outer  group  of 
four  large  ones  is  a  wide  gap,  in  which  hundreds  of 
very  small  planets  are  revolving  about  the  sun.    These 
are  called  Planetoids,  or,  better,  Minor  Planets.1    Their 
number  is  unknown,  and  may  be  very  great;  but  the 
mass  of  all  of  them  put  together  is  much  less  than 
that  of  the  smallest  of  the  principal  planets.      The 
orbits  of  these  planets  are  much  more  elliptical  than 
those  of  the  larger  ones ;  some  of  them  are  twice  as 
far  from  the  sun  at  aphelion  as  at  perihelion.     And 
while  the  orbits  of  the  principal  planets  all  make  small 
angles  with  the  ecliptic,  the  orbits  of  some  of  the 
minor  planets  make  large  angles  with  it.     Their  dis- 
tances  from  the   sun   vary   greatly.     Some   of  them 
almost  intersect  the  orbit  of  Mars,  others  swing  out 
nearly  as  far  as  Jupiter's  orbit,  while  the  whole  space 
between  is  filled  with  them.     But  their  orbits  are  so 
entangled  that,  if  they  were  actual  rings,  scarcely  one 
of  them  could  be  picked  up  without  disturbing  all  the 
rest. 

154.  The  Discovery  of  the  Minor  Planets. — The  gap 
between  Mars  and  the  outer  group  of  planets  was  long 


1  Also  often  called  asteroids  (star-like)   but  the  term  is  gradually 
giving  way  to  the  more  appropriate  one  of  minor  planets. 


160  ASTRONOMY. 


since  noticed  by  astronomers.  Besides,  a  law,  known 
as  Bode's  law,1  had  been  devised,  which,  if  this  gap 
were  only  filled  by  a  planet  in  its  proper  place,  ex- 
pressed quite  accurately  the  relative  distances  of  the 
known  planets  from  the  sun.  These  facts  led  astrono- 
mers, about  the  beginning  of  this  century,  to  make 
careful  search  for  the  missing  planet.  Twenty-four 
astronomers  divided  the  zodiac2  into  as  many  parts, 
and  began  the  search.  But  on  the  first  day  of  the 
present  century,3  before  they  got  fairly  started,  an  Ital- 
ian astronomer,  an  outsider,  discovered  a  small  planet 
in  the  vacant  space.  In  seven  years  three  more  were 
found,  but  after  that  na  more  until  1845.  Since  that 
time  a  large  number  have  been  found,  and  sometimes 
ten  or  twelve  are  discovered  in  a  year.  The  number 
already  known  is  about  four  hundred  and  fifty. 

1  If  the  series  0,  3,  6,  12,  24,  48,  96,  etc.,  be  formed  by  doubling 
each  term  after  the  first  to  get  the  next  one,  and  then  4  be  added  to 
each  term,  we  shall  have  4,  7,  10,  16,  28,  52,  100,  etc.     If  we  multi- 
ply each  of  the  first  four  of  these  numbers  by  9,000,000,  we  shall  have 
pretty  nearly  the  distances  of  the  inner  group  of  planets  from  the 
sun.     Fifty-two  multiplied   by   the  same  number  gives   about  the 
distance  of  Jupiter,  the  first  of  the  outside  group,  while  28  times 
9,000,000  was  supposed  to  be  the  distance  of  the  undiscovered  planet. 
This  is  called  Bode's  law,  after  a  celebrated  German  astronomer  who 
died  in  1826,  but  it  was  devised  long  before  his  time  by  Titius.     The 
law  held  good  for  all  the  principal  planets  until  the  discovery  of 
Neptune,  the  outermost  of  the  outside  group,  in  1846,  for  which  it 
was  found  to  fail.     These  coincidences  are  now  believed  to  be  merely 
accidental. 

2  It  will  be  remembered  that  the  zodiac  is  a  belt  of  the  sky  ex- 
tending about  8°  on  each  side  of  the  ecliptic,  in  which  all  the  prin- 
cipal planets  are  always  found.     It  was  therefore  very  natural  that 
they  should  examine  the  zodiac  for  the  new  planet.     But  the  minor 
planets,  as  we  now  know,  are  not  all  within  the  zodiac. 

*  What  day  was  this  ?    See  page  14,  note  3. 


THE  MINOR  PLANETS.  161 

The  discovery  of  these  planets  is  a  difficult  task. 
None  that  are  now  discovered  can  be  seen  with  the 
naked  eye,  and  in  the  best  telescopes  they  look  exactly 
like  very  small  stars,  from  which  they  can  be  distin- 
guished only  by  their  motions  among  the  stars.  Among 
the  astronomers  who  have  occupied  themselves  largely 
with  this  work,  two  Americans,  Profs.  Peters1  and 
Watson,2  and  Palisa,  an  Austrian,  have  been  most 
successful.  The  new  planets  whose  discovery  we  see 
occasionally  announced  in  the  newspapers  are  always 
some  of  these. 

The  first  minor  planets  found  were  named  after 
various  goddesses,  and  this  custom  is  still  adhered  to. 
But  their  great  number  has  made  it  very  difficult  to 
find  enough  such  names  for  them,  so  that  many  of 
those  lately  discovered  have  not  yet  been  named. 
They  are  usually  designated  simply  by  numbers,  which 
show  the  order  of  their  discovery,  the  numbers  being 
enclosed  in  circles,  thus :  @,  Q,  @. 

155.  Description  of  the  Minor  Planets. — As  has  been 
said,  these  planets  are  very  small.  Two  or  three  of 
them  may  rarely  and  under  the  most  favorable  circum- 
stances be  seen  by  the  naked  eye,  but  only  as  very  faint 
stars.  Many  of  them  can  be  seen  only  through  good 
telescopes.  None  of  them  have  any  apparent  size  even 
in  the  largest  telescopes,  so  that  their  size  can  be  esti- 
mated only  from  the  amount  of  light  they  give.  The 
largest  of  them  may  be  three  hundred  miles  in  diam- 
eter, while  the  smallest  yet  discovered  is  probably  not 


1  C.  H.  F.  Peters,  1813-1890,  Professor  of  Astronomy  at  Hamilton 
College,  Clinton,  New  York. 

2  See  page  65,  note  3. 

I  14* 


162  ASTRONOMY. 


more  than  fifteen  miles  in  diameter.  On  these  planets 
the  attraction  of  gravity  is  slight,  and  one  would  there- 
fore weigh  much  less  than  upon  the  earth.  Sir  John 
Herschel  remarks  that  "  a  man  placed  upon  one  of 
the  minor  planets  would  spring  with  ease  sixty  feet, 
and  sustain  in  his  descent  no  greater  shock  than  he 
does  on  the  earth  from  leaping  a  yard."  Of  their  rota- 
tion, surface,  atmosphere,  etc. ,  we  know  nothing.  Their 
appearance  presents  no  features  of  interest  in  any  tele- 
scope. 

156.  Are  the  Minor  Planets  Fragments  of  one  Large 
Planet? — When  the  first  two  or  three  of  these  planets 
had  been  found,  it  was  suggested  that  they  might  be 
fragments  of  a  larger  planet  which  had  from  some 
cause  burst  to  pieces.  The  theory  at  first  seemed 
probable,  but  it  has  been  long  since  rejected  by  as- 
tronomers. So  far  as  we  can  tell,  they  have  been  re- 
volving about  the  sun  ever  since  the  solar  system  was 
created.1 

A  new  asteroid  (No.  433),  discovered  in  1898,  seems 
to  be  the  nearest  heavenly  body  to  the  earth,  except 
the  moon.  Its  mean  distance  from  the  sun  is  only 
about  one  and  a  half  the  earth's  distance,  and  under 
most  favorable  conditions  it  may  approach  within 
13,000,000  miles.  It  will  then  appear  as  a  star  of  the 
sixth  magnitude,  and  may  be  the  best  of  all  bodies  by 
which  to  determine  the  parallax  of  the  sun. 

1  And  yet  the  facts  of  their  revolving  about  the  sun  all  in  a  ring- 
together,  and  of  having  their  orbits  so  entwined,  seem  to  indicate 
some  connection  among  them  originally.  Perhaps  we  might  regard 
them  as  a  group  of  great  meteoroids  revolving  about  the  sun,  just  as 
many  groups  of  small  meteoroids  (shooting-stars)  are  now  known  to 
be  revolving  about  the  sun  (Art.  197),  and  just  as  Saturn's  rings  ai^e 
probably  dense  groups  of  small  meteoroids  revolving  about  him  (Art 
169). 


JUPITER.  163 


JUPITEK.    % 

Distance  from  the  Sun,  480,000,000  Miles.  Diameter,  86,500 
Miles.  Length  of  Year,  12  Years.  Length  of  Day,  10  Hours. 
Specific  Gravity,  1£.  Five  Satellites. 

157.  Jupiter's  Relations  to  the  Solar  System. — The  first 
of  the  outside  group  of  four  planets  is  Jupiter.1  He  is 
the  largest  of  all  the  planets,  his  volume2  being  one 
and  a  half  and  his  mass2  two  and  a  half  times  as  great 
as  that  of  all  the  other  planets  put  together.  Com- 
pared with  Jupiter,  our  earth  is  insignificant.  It  would 
take  about  1400  earths  to  make  a  planet  as  large  as 
Jupiter,  although  300  earths  would  make  one  as  heavy 
as  he,  for  Jupiter's  specific  gravity  is  less  than  one- 
fourth  of  the  earth's.  Yet  his  volume  and  mass  are 
only  Y^  ff  of  those  of  the  sun.  At  his  great  distance, 
one  would  receive  from  the  sun  only  about  -fa  as  much 
heat  and  light  as  upon  the  earth.  If  he  depends  solely 
upon  the  sun  for  heat,  the  temperature  of  his  surface 
must  be  nearly  500°  below  zero.  Notwithstanding  his 
great  size,  Jupiter's  day  is  not  half  as  long  as  ours. 
His  year  is  as  long  as  twelve  of  ours,  but,  as  his  equator 
makes  an  angle  of  only  3°  with  his  orbit,3  he  has  no 
changes  of  seasons  of  any  consequence. 

1  Ju'pi-ter,  the  chief  of-  the  gods.  His  symbol  (%)  is  perhaps  a 
rude  representation  of  an  eagle,  the  bird  of  Jupiter. 

8  The  student  must  be  careful  to  have  a  very  clear  notion  of  what 
these  mean.  Volume  is  size  or  bulk ;  it  varies  according  to  the  cube 
of  the  diameter.  Mass  is  quantity  of  matter  ;  it  varies  according  to 
the  weight.  Then  if  all  of  the  planets  could  be  weighed  at  the  same 
place  (at  the  sun,  for  instance),  Jupiter  would  weigh  two  and  a 
half  times  as  much  as  all  of  the  rest  together. 

8  How  many  degrees  wide  is  Jupiter's  torrid  zone  ?  his  frigid  and 
temperate  zones  ? 


164  ASTRONOMY. 

158.  Jupiter's  Motions  and  Phases. — Jupiter  is  so  far 
off  that  he  shows  no  phases  to  the  ordinary  ohserver. 
His  disk  in  the  telescope  is  always  full.  But  the  great 


FIG.  37.— JUPITER,  AS  SEEN  IN  THE  TELESCOPE  or  HAVERFORD  COLLEGE  OBSER- 
VATORY (INVERTED),  JANUARY  7, 1882;  SHOWING  THE  SHADOW  OF  A  SATELLITE  ON 
ITS  DISK,  ANOTHER  SATELLITE  JUST  EMERGING  FROM  IN  FRONT,  AND  THE  OVAL 
"  RED  SPOT." 

velocity  of  his  rotation  upon  his  axis  makes  him  no- 
ticeably flattened  at  the  poles.  Like  Mars  and  all  the 
other  superior  planets,  Jupiter  has  a  retrograde  motion 
among  the  stars  when  he  is  at  opposition.  This 
occurs  once  in  about  thirteen  months. 


JUPITER.  165 


159.  Jupiter's  Belts. — When  seen  through  a  telescope, 
the  most  conspicuous   feature  of  Jupiter  is  his  belts. 
These  are  dark  bands  or  streaks  stretching  across  the 
planet.     Usually  there  are  two  conspicuous  ones,  just 
above  and  below  the  planet's  equator.     But  others  are 
often  seen,  sometimes  covering  the  greater  part  of  his 
surface.     They  are  well  shown  in  Fig.  37.     Sir  Wil- 
liam Herschel l  thought  that  the  belts  were  openings  in 
the  planet's  atmosphere,  through  which  we  can  see  the 
darker  surface  of  the  planet  itself.   Jupiter  is  certainly 
surrounded  by  an  atmosphere,  but  so  dense  and  deep 
is  it  that  it  is  probable  that  we  never  see  the  planet's 
surface.     The  belts  seem  to  be  fissures  in  the  upper 
clouds  and   atmosphere,  through   which   we   see  the 
lower  strata   of   atmosphere   and  clouds,  which  are 
darker  because  they  reflect  less   sunlight.     In  small 
telescopes  the  belts  are  simply  dark,  but  in  better  in- 
struments they  often  display  colors.    This  color  is  usu- 
ally a  dull  red,  but  occasionally  varied  and  more  bril- 
liant ones  have  been  seen.     The  cause  of  these  colors 
and  their  changes  is  not  known. 

160.  Spots  on  Jupiter. — Besides  the  ever-present  belts, 
spots  are  also  sometimes  seen  upon  Jupiter's  surface. 
They  are  commonly  dark  or  of  a  dull-red  color,  but 
bright  white  ones  have  been  seen.    The  dark  ones  may 
be  openings  in  the  upper  atmosphere,  while  the  white 
ones  look  somewhat  like  the  round  masses  of  white 
cloud  which  are  so  common  here  in  summer.    Some  of 
these  spots  have  remained  visible  for  a  considerable 
time,  and  it  is  by  observations  of  these  that  the  period 

1  Born  1738,  died  1822.  A  German  by  birth,  but  spent  all  of  his 
manhood  in  England.  Generally  conceded  to  be  the  greatest  prac- 
tical astronomer  that  has  yet  lived. 


166  ASTRONOMY. 

of  the  planet's  rotation  has  been  determined.  The 
various  observations  show  that  these  spots  have  some 
motion  of  their  own,  so  that  it  is  not  possible  to  deter- 
mine the  period  of  Jupiter's  rotation  as  exactly  as  that 
of  Mars.  A  very  large  spot,  known  as  "  the  red  spot," 
which  appeared  in  1878,  remained  for  several  years  about 
the  same  shape  and  size.  Then  it  gradually  faded  away, 
and  again  reappeared.  It  is  in  the  southern  hemisphere 
of  the  planet,  rather  oval  in  shape,  about  24,000  miles 
long,  and  of  a  dull-red  color.  It  appears  to  be  quite 
permanent,  though  showing  changes  of  outline,  color, 
and  intensity.1 

161.  Jupiter's  Temperature. — There  is  strong  evidence 
that  Jupiter  is  still  very  hot.     The  sun's  rays  are  too 
feeble  there  to  raise  the  thick  clouds  which  constantly 
envelop  him,  and  the  great  changes  continually  going 
on  in  his  atmosphere  must  be  caused  by  intense  heat 
within.      The   earth  shows   plainly  that  it  wras  once 
in  a  molten  state,  and  if  it  was  created  at  the  same 
time  as  Jupiter,  the  great  mass  of  the  latter  would 
keep  him  hot  long  after  the  earth  had  cooled  off.     If 
the  body  of  the  planet  has  yet  solidified,  it  is  still  prob- 
ably white-hot.      So  that  Jupiter  is  more  like  the  sun 
than  like  the  earth.     And  indeed  there  is  evidence 
that  he  actually  gives  out  some  light  of  his  own,  even 
through  his  dense  cloudy  atmosphere.    The  amount  of 
this,  however,  if  any,  cannot  be  great :  the  most  of  his 
light  is  reflected  sunlight. 

162.  Jupiter's   Satellites.— Among   the   first    objects 
which  Galileo's  little  telescope  revealed  to  him  were 

1  As  determined  by  observations  on  this  spot,  Jupiter  rotates  in 
about  9  h.  55  m.  35  sec. 


JUPITER.  167 


four  moons  revolving  around  Jupiter.  They  are  almost 
bright  enough  to  be  seen  by  the  naked  eye,  and  indeed 
they  have  on  rare  occasions  been  thus  seen  by  persons 


Fio.  38.— JUPITER  AND  HIS  SATELLITES. 


having  exceptionally  good  eyesight.  Were  it  not  for 
the  overpowering  brightness  of  Jupiter,  they  would 
be  usually  visible.  The  satellites  are  known  by  their 
numbers,  the  one  nearest  the  planet  being  called  the 
first,1  the  next  one  the  second,1  and  so  on.  The  near- 
est one  is  a  little  farther  from  Jupiter  than  the  moon 
is  from  the  earth,  the  fourth  is  more  than  a  million 
miles  off.  The  second  is  the  smallest,  and  is  about 
the  size  of  our  moon ;  the  third,  which  is  the  largest, 
is  about  3700  miles  in  diameter,  being  considerably 
larger  than  Mercury.  They  all  revolve  about  Jupiter 
in  the  same  direction  that  the  planets  revolve  about 
the  sun,  from  west  to  east.  The  first  makes  his  revo- 
lution in  If  days,  the  fourth  in  less  than  17  days. 
Their  orbits  are  all  nearly  circular,  and  lie  almost  in 
the  plane  of  Jupiter's  orbit.  On  this  account  they 


1  They  are  generally  designated  by  the  Koman  numerals,  I.,  II., 
III.,  IV. 


168  ASTRONOMY. 


are  never  far  out  of  a  straight  line  passing  through 
Jupiter  (Fig.  38). 

163.  Eclipses  and  other  Phenomena  of  Jupiter's  Satellites. 
— Jupiter's  moons,  like  our  own,  are  eclipsed,  but, 
owing  to  the  size  of  the  planet's  shadow  and  the  coin- 
cidence of  their  orbits  with  the  plane  of  Jupiter's  orbit, 
they  are  eclipsed  much  oftener  than  our  moon.  The 
three  inner  satellites  are  eclipsed  at  every  revolution, 
and  the  outer  one  generally  is.  In  Fig.  39  the  satellite 


FIG.  39.— PHENOMENA  or  JUPITER'S  SATELLITES. 

is  eclipsed  while  passing  from  1  to  2.  It  is  then  en- 
tirely invisible,  which  proves  that  if  Jupiter  emits  any 
light  of  his  own  it  must  be  very  little.  From  3  to  4 
the  satellite  is  again  invisible,  because  it  is  behind  the 
planet.  This  is  an  occultation  of  the  satellite.  1  and  3 
are  the  points  of  disappearance,  2  and  4  of  reappearance. 
The  orbit  of  the  first  satellite  in  the  figure  shows  that 
the  occultation  may  begin  before  the  eclipse  ends.  On 
the  opposite  side  of  its  orbit,  from  5  to  6,  the  satellite 
is  between  the  sun  and  Jupiter,  and  partly  eclipses  the 
planet.  The  shadow  of  the  satellite,  a  small  round 
black  spot,  is  seen  crossing  Jupiter's  disk.  From  7  to 
8  the  satellite  transits  across  the  planet,  and  in  a  good 


JUPITER.  169 


telescope  maybe  seen,  being  sometimes  a  little  brighter 
and  at  other  times  a  little  darker  than  the  surface  of 
the  planet  behind  it.  Sometimes  the  satellite  and  its 
shadow  will  be  seen  upon  Jupiter  at  the  same  time,  as 
the  orbit  of  the  first  satellite  in  the  figure  shows.  The 
passing  of  the  satellite  or  shadow  on  the  edge  of  the 
planet  is  called  ingress  ;  the  passing  off,  egress. 

It  was  by  observations  of  Jupiter's  satellites  that  the 
velocity  of  light  was  first  found.  See  Art.  239. 

164.  Observations  of  Jupiter. — Next  to  Yenus,  Jupiter 
is  the  brightest  of  the  planets,  and  of  course  far  brighter 
than  any  of  the  fixed  stars.  But  while  Yenus,  being 
an  inferior  planet,  is  never  seen  far  from  the  sun,  and 
never  at  a  late  hour  of  the  night,  Jupiter,  being  a  su- 
perior planet,  may  be  at  any  distance  from  the  sun, 
and  may  shine  all  night.  These  facts,  together  with 
the  information  given  about  the  times  when  it  is  an 
evening  and  when  a  morning  star,  will  usually  enable 
one  to  recognize  Jupiter.  And  when  once  found,  one 
may  keep  track  of  him  from  year  to  year,  for,  on  ac- 
count of  the  great  length  of  his  orbit,  he  moves  but 
slowly  among  the  stars.  As  he  takes  twelve  years  to 
journey  around  the  sun,  he  moves  through  one  con- 
stellation in  the  zodiac  each  year.  Although  Jupiter 
is  always  bright,  and  can  always  be  observed  with 
interest,  except  when  too  near  the  sun,  yet  he  is  of 
course  brightest  and  best  seen  when  in  opposition. 
With  a  telescope  which  magnifies  sixty  times  its  di- 
ameter will  seem  as  large  as  that  of  the  full  moon. 
With  larger  and  more  powerful  glasses  it  becomes 
an  object  of  great  interest,  and  the  surface  of  the 
planet,  as  it  slowly  revolves,  may  be  studied  night 
after  night  with  profit.  Through  even  a  very 

15 


170  ASTRONOMY. 


small  telescope  Jupiter  is  interesting,  and  his  moons 
may  be  seen  through  a  small  spy-glass  or  a  common 
opera-glass.  They  look  like  small  stars,  and  will  be 
easily  distinguished  by  their  being  about  in  a  straight 
line  with  -the  planet.  Generally  one  or  two  will  be 
on  one  side,  and  the  others  on  the  other ;  rarely  all 
four  will  be  seen  on  one  side,  and  sometimes  one  or 
two  may  be  invisible  from  being  eclipsed  or  in  occul- 
tation.  Under  favorable  circumstances  (see  p.  290),  a 
two-inch  telescope  will  show  the  principal  belts  of  Jupi- 
ter, and  in  larger  instruments  the  other  belts  and  spots 
may  be  seen.  Great  activity  is  often  manifest  upon 
Jupiter,  and  all  who  can  do  so  should  watch  it,  ob- 
serving the  period  of  rotation  from  the  spots,  noting 
the  colors  and  any  changes  or  peculiarities,  and  mak- 
ing maps  of  its  surface.  When  a  spot  can  be  ob- 
served on  Jupiter,  those  having  suitable  instruments 
ought  to  note  the  times  of  the  passage  of  the  spot  over 
the  planet's  central  meridian  for  as  long  a  time  as  pos- 
sible, to  determine  the  period  of  the  planet's  rotation 
In  all  astronomical  observations  the  time  must  be  care- 
fully noted. 

165.  Observations  of  Jupiter's  Satellites. — For  telescopes 
of  moderate  size  the  most  interesting  observations  of 
Jupiter  are  those  upon  the  phenomena  of  his  satel- 
lites. All  of  the  phenomena  described  in  Art.  163 
are  predicted  in  the  Nautical  Almanac,1  and  may  be 

1  The  Nautical  Almanac  is  published  by  the  United  States  gov- 
ernment at  Washington,  D.C.,  for  each  year,  and  comes  out  about 
three  years  in  advance.  It  is  indispensable  to  the  navigator  and  the 
astronomer.  It  may  be  obtained  by  sending  one  dollar  to  the  Bureau 
of  Navigation,  Washington,  D.C. 

The  following  data  are  taken  from  page  460  of  the  Almanac  for 


JUPITER. 


171 


observed  at  the  times  given  there.1  The  Nautical 
Almanac  also  gives  figures  showing  the  positions  of 
the  satellites  for  every  day  in  the  year,  by  which  the 
different  satellites  can  be  distinguished.  For  the  ap- 
parent order  of  the  distances  of  the  satellites  from  the 
planet  may  not  be  the  real  order  of  their  distances.  If 
Satellite  IV.  were  almost  in  front  of  or  behind  Jupiter, 
it  might  appear  to  be  the  nearest  of  all.  Satellite  III. 
may  generally  be  distinguished  by  its  size. 

When  a  transit  or  occultation  is  to  be  observed,  every- 
thing should  be  ready  at  least  five  minutes  before  the 
predicted  time ;  and  the  error  of  the  watch  which  is 
to  be  used  for  taking  the  time  must  be  known.  In  an 
eclipse,  the  times  of  the  first  diminution  of  brightness 

1883.  They  give  the  time  of  the  phenomena  about  the  period  of 
Jupiter's  -opposition  in  1883. 

DECEMBER,  1883. 


Date. 

No.  of 
Satellite. 

Phenomenon. 

Phase. 

d.     h.     m.      s. 

21     10    45 

I. 

Shadow. 

Ingress. 

11     26 

I. 

Transit. 

Ingress. 

13      5 

I. 

Shadow. 

Egress. 

13    43 

III. 

Occultation. 

Reappearance. 

13    46 

I. 

Transit. 

Egress. 

22      7    57      6.1 

I. 

Eclipse. 

Disappearance. 

10    55 

I. 

Occultation. 

Reappearance. 

14    12    30.4 
18     26 

II. 
II. 

Eclipse. 
Occultation. 

Disappearance. 
Reappearance. 

23      7    34 

I. 

Shadow. 

Egress. 

8    13 

I. 

Transit. 

Egress. 

24      8    25 

II. 

Shadow. 

Ingress. 

9    41 

II. 

Transit. 

Ingress. 

11     19 

II. 

Shadow. 

Egress. 

("  Shadow,"  in  the  third  column,  means  the  transit  of  the  satellite's 
shadow  across  Jupiter's  disk.) 

1  The  dates  in  the  Almanac  are  in  Washington  astronomical  time. 
These  must  he  reduced  to  civil  time  (see  Art.  89),  and  then  the  cor- 
responding local  time  found,  as  in  Art.  102.  The  dates  given  may  be 
one  or  two  minutes  in  error. 


172  ASTRONOMY. 


and  of  final  disappearance  should  be  noted.  At  reap- 
pearance the  order  will  be  reversed,  the  times  of  first 
reappearance  and  of  complete  restoration  of  brightness 
being  noted.  Two  persons  can  take  time  better  than 
one :  while  one  observes,  the  other  can  consult  the  watch. 
In  occultations  the  time  of  first  contact,  that  is,  when  the 
satellite  first  touches  the  edge  of  the  planet,  and  that  of 
last  contact,  when  the  last  part  of  the  satellite  disappears 
behind  the  planet,  should  be  noted  if  the  telescope  is 
large  enough  to  show  them.  The  transits  of  the  satel- 
lites and  the  shadows  are  more  interesting,  but  require 
rather  better  telescopes.  To  see  them  well,  a  four-inch 
glass  is  needed.  The  transit  of  the  shadow  is  seen  more 
easily  than  that  of  the  satellite,  although  the  times  of 
ingress  and  egress  are  harder  to  determine  than  those 
of  the  satellite.  Sometimes  a  satellite  and  its  shadow 
may  be  seen  upon  the  planet  at  the  same  time.  The 
first  and  last  contacts  in  a  satellite's  transit,  like  those 
of  the  occupation,  should  be  noted.  If  the  state  of  the 
atmosphere  is  such  that  the  exact  times  cannot  be  deter- 
mined, they  may  be  carefully  estimated.  Records  of 
all  observations,  with  notes  upon  the  state  of  the  atmos- 
phere and  the  reliability  of  the  observations,  should  be 
kept.1 

1  Fig.  39  shows  that  when  the  earth  is  at  E  the  eclipses  come  before 
the  occultations,  and  the  transits  of  the  shadows  before  the  transits  of 
the  satellites.  When  the  earth  is  in  this  part  of  its  orbit,  Jupiter 
rises  more  than  twelve  hours  after  the  sun,  and  therefore  in  general 
rises  in  the  night.  But  when  the  earth  is  at  F  in  the  figure,  the  oc- 
cultations and  transits  of  satellites  come  first.  And  when  the  earth 
is  in  this  part  of  its  orhit,  Jupiter  rises  less  than  twelve  hours  after 
the  sun,  or  in  general  in  the  daytime.  The  following  approximately 
correct  rule  may  he  derived  from  these  facts  :  If  Jupiter  rises  in  the 
night,  the  eclipses  will  precede  the  occultations,  and  the  shadows  will 


SATURN.  173 


In  ordinary  telescopes  the  satellites  will  be  simply 
points  of  light  like  the  stars.  In  large  instruments 
their  disks  can  be  seen,  and  some  curious  and  unex- 
plained changes  in  their  appearance  have  been  ob- 
served. 

A  fifth  satellite  to  Jupiter  was  discovered  by  Pro- 
fessor E.  E.  Barnard  at  the  Lick  Observatory,  Cali- 
fornia, in  1892.  It  can  be  seen  by  only  the  largest 
telescopes.  It  is  very  small,  is  only  about  112,000 
miles  from  the  centre  of  the  planet,  and  performs  its 
revolution  in  about  twelve  hours. 


SATURN,    h 

Distance  from  the  Sun,  890,000,000  Miles.  Diameter,  73,000 
Miles.  Length  of  Year,  30  Years.  Length  of  Day,  10  Hours. 
Specific  Gravity,  f .  Nine  Satellites. 

166.  Relations  to  the  Solar  System.  —  Next  beyond 
Jupiter,  but  almost  twice  as  far  from  the  sun,  is  Saturn.1 
Next  to  Jupiter  he  is  the  largest  of  the  planets.  His 
volume  is  700  times  that  of  the  earth.  But  his  mass 
is  only  about  100  times  as  great,  which  shows  that 
Saturn  must  be  made  of  very  light  material.  His 
specific  gravity  is  the  least  of  all  the  planets,  and  he  is 
only  three-fourths  as  heavy  as  a  globe  of  water  of  his 
size.  He  would  float  in  water.  What  has  been  found 
to  be  true  of  the  densities  of  Jupiter  and  Saturn  is  also 

cross  the  planet  ahead  of  the  satellites  ;  but  if  Jupiter  rises  in  the 
daytime,  in  which  case  it  will  be  found  up  some  distance  in  the  sky 
in  the  evening,  the  occultations  and  transits  of  the  satellites  will  come 
first. 

1  Saturn,  the  god  of  time,  father  of  Jupiter.  His  symbol  (h)  repre- 
sents an  old-fashioned  scythe  or  sickle. 

15* 


174  ASTRONOMY. 


true  of  the  other  two  outside  planets.  None  of  them 
differ  much  from  water  in  density ;  all  are  much  lighter 
than  the  four  inner  planets.  But  in  all  of  these  we  see 


FIG.  40.— SATURN. 


and  measure  the  outside  of  their  atmospheres,  so  that 
what  we  give  as  the  specific  gravity  of  the  planet  is 
really  the  average  specific  gravity  of  the  planet  and  its 
atmosphere  taken  together.  If  the  atmosphere  is  deep, 
the  real  body  of  the  planet  is  smaller  and  denser.  The 
size  and  density  of  the  real  planet  we  cannot  determine, 
because  it  cannot  be  seen  through  the  atmosphere. 
Since  Saturn  is  nearly  twice  as  far  from  the  sun  as 
Jupiter,  one  would  there  receive  only  about  one-fourth 
as  much  light  and  heat  from  the  sun  as  upon  Jupiter. 
167.  Description  of  Saturn. — Saturn's  enormous  dis- 


SATURN.  175 


tarice  from  us  makes  it  impossible  far  us  to  see  much  of 
his  surface.  But,  so  far  as  we  can  tell,  the  body  of  the 
planet  is  very  much  like  Jupiter.  He  has  faint  belts, 
and  probably  he  has  not  yet  cooled  off  into  a  solid 
body  like  the  earth,  and  is  surrounded  by  a  dense  at- 
mosphere and  clouds.  Spots  are  sometimes  seen  upon 
his  surface,  and  when  seen  have  been  used  to  determine 
the  time  of  his  rotation  upon  his  axis.  He  is  even  more 
flattened  at  the  poles  than  Jupiter. 

168.  Saturn's  Rings. — The  most  remarkable  feature 
about  Saturn,  and  one  possessed  by  no  other  body  in 
the  solar  system,  is  a  set  of  enormous  rings  surround- 
ing him.  Fig.  40  well  represents  these  remarkable 
rings,  and  also  the  planet  itself.  Through  a  small  tele- 
scope one  bright  ring  is  seen,  but  a  larger  instrument 
shows  that  this  ring  is  divided  into  two,  one  within  the 
other.  The  inner  ring  is  the  wider  of  the  two.  In 
1850,  Prof.  Bond1  discovered  a  very  faint  third  ring 
inside  of  the  others,  a. continuation  of  the  inner  bright 
ring;  it  is  commonly  called  the  dusky  ring.  The  di- 
ameter of  the  rings  from  outside  to  outside  is  about 
170,000  miles,  and  the  two  bright  rings  together  are 
some  30,000  miles  wide.2  The  dusky  ring  extends 
about  half-way  from  the  inner  bright  ring  to  the  body 
of  the  planet  (shown  in  Fig.  40).  The  rings  are  very 
thin,  probably  not  more  than  100  miles  through.  A 
ring  of  good  writing-paper  one  foot  in  diameter  will 


1  G.  P.  Bond  (1826-1865),  associate  of,  and  successor  to,  his  father, 
W.  C.  Bond  (1789-1859),  at  Harvard  Observatory. 

2  How  far  is  the  inner  edge  of  the  bright  rings  from  the  body  of  the 
planet  ? 

The  division  between  the  two  bright  rings  is  less  than  2000  miles 
wide. 


176  ASTRONOMY. 


represent  their  proportions  pretty  correctly.  Other  di- 
visions of  the  rings  have  been  suspected  by  astrono- 
mers, but  if  any  have  been  seen  they  must  have  been 
but  temporary  ones. 

169.  The   Constitution   of  the  Rings.  —  Mathematical 
reasoning1  has  shown  that  in  all  probability  the  rings 
are   composed  of  small,  distinct  particles  of  matter, 
too  small  to  be  seen  separately,  and  so  close  together 
that  we  cannot  see  through  them ;   just  as  a  column 
of  smoke  or  a  cloud  looks  like  one  solid  mass,  when  it 
is  really  made  up  of  a  great  many  little  particles  of 
matter  crowded  together.     In  the  dusky  ring  the  par- 
ticles may  not  be  crowded  together  so  thickly  as  in  the 
bright  rings.     The  rings  shine  by  reflecting  the  sun's 
light.    The  particles  composing  the  rings  must  revolve 
about  the  planet,  or  they  would  fall  to  his  surface. 
The  rings  are  then  really  a  cloud  of  small  satellites 
chasing   each   other   around  in  a  ring   about  Saturn. 
The  time  of  rotation  is  thought  to  be  a  little  greater 
than  that  of  the  planet. 

170.  Appearances  of  Saturn's  Rings. — The  rings  are 
not  perpendicular  to  the  earth's  orbit,  but  inclined  to 
it  at  an  angle  of  27°.     We  never,  therefore,  get  a  full 
front  view  of  the  rings ;  when  widest  open,  they  seem 
to  us  like  rather  narrow  ellipses.     Twice  during  Sat- 
urn's revolution  about  the  sun,  or  about  every  fifteen 
years,  the  edge  of  the  ring  is  towards  the  earth.     So 


1  Prof.  Benjamin  Peirce  (1809-1880),  of  Harvard  University, 
proved  that  the  rings  could  not  be  continuously  solid,  and  thought 
they  were  probably  liquid.  But  Prof.  J.  Clerk  Maxwell  (1831- 
1879),  of  Cambridge  University,  England,  proved  that  they  could 
not  be  liquid,  and  hence  inferred  their  probable  constitution  to  be  aa 
given  above. 


SATURN. 


177 


thin  is  the  ring  that  at  this  time  it  is  entirely  invisible 
in  ordinary  telescopes,  and  Saturn  seems  to  be  simply 
a  round  planet  like  the  rest.  In  powerful  telescopes 
the  ring  at  this  time  looks  like  a  fine  wire  running 
through  the  centre  of  the  planet.  This  occurred  in 
February,  1878,  and  will  occur  again  in  December, 


Fio.  41.— THE  DIFFERENT  APPEARANCES  OF  SATURN'S  RINGS. 


1891.  After  passing  this  point,  the  ring  gradually 
opens  wider  and  wider,  until,  when  half-way  to  the 
next  disappearance,  we  see  it  at  an  inclination  of  about 
27°,  the  most  favorable  position  for  its  observation. 
As  the  figure  shows,  this  is  its  position  in  1885  and  in 
1899. 

171.  Saturn's  Satellites. — Saturn  has  nine  satellites, 
— twice  as  many  as  any  other  planet.  The  nearest  is 
about  half  as  far  from  Saturn  as  the  moon  is  from  the 


178  ASTRONOMY. 


earth,  the  farthest  is  more  than  2,000,000  of  miles 
away.  The  sixth  satellite  is  the  largest,  being  over 
3000  miles  in  diameter ;  the  smallest  ones  are  too  small 
for  measurement.  The  sixth  can  be  seen  in  any  tele- 


FIG.  42. — SATURN  AND  HIS  SATELLITES. 

scope.  The  eighth  is  as  bright  as  the  sixth  when  west 
of  the  planet,  but  can  be  seen  only  in  large  telescopes 
when  east  of  it.  It  is  supposed  that  one  side  of  it  is 
much  darker  in  color  than  the  other,  and  as  it  turns  on 
its  axis  the  bright  and  dark  sides  are  turned  towards  us 
alternately.  From  the  fact  that  it  always  disappears 
upon  the  same  side  of  Saturn,  it  is  inferred  that,  like 
our  moon,  it  rotates  once  on  its  axis  during  each  revo- 
lution about  the  planet.  The  smallest  satellites  are 
visible  only  in  the  largest  telescopes. 

The  satellites  revolve  about  Saturn   nearly  in  the 


SATURN.  179 


plane  of  the  rings, — at  an  angle,  therefore,  of  nearly  27° 
to  Saturn's  orbit  as  well  as  to  our  own ;  for  Saturn's 
orbit  is  nearly  in  the  same  plane  as  the  earth's.  Hence 
they  are  seldom  eclipsed.  In  passing  around  the  planet 
they  generally  cross  above  or  below  the  shadow.  When 
they  do  occur,  on  account  of  their  great  distance  and 
the  small  size  of  most  of  the  satellites,  the  eclipses  and 
transits  are  of  little  interest. 

172.  Vieivs  from  Saturn. — If  an  observer  on  Saturn 
could   see  through   its   atmosphere,  the  view  of  the 
heavens  must  be  striking.      Although  the  sun  there 
has  but  y1^  of  the  diameter  that  he  has  to  us,  and  sheds 
then  scarcely  -^  of  the  light  and  heat  that  we  get,  yet 
the  eight  moons  and  the  wonderful  rings  would  be  an 
interesting  spectacle.     The  rings  form  broad,  bright, 
rainbow-shaped  arches  crossing  the  heavens,  each  side 
being  bright  and  dark  alternately  for  fifteen  years  at 
a  time.     The  rings  must  cause  frequent  and  long-con- 
tinued eclipses  of  the  sun. 

173.  Observation  of  Saturn. — To  the  naked  eye  Saturn 
is  not  an  object  of  much  interest.     It  is  not  nearly  so 
bright  as  Venus  or  Jupiter,  and  is  surpassed  in  bright- 
ness by  one  or  two  of  the  fixed  stars.     On  account  of 
its  distance,  its  brightness  does  not  vary  much,  although 
it  is  somewhat  increased  when  the  rings  are  opened 
wide.    Hence  it  is  not  so  easily  found  as  the  brighter 
planets ;  but  the  data  given  on  the  first  page  and  in  the 
body  of  an  almanac,  together  with  the  fact  that  it  is  a 
strange  star  in  the  constellation1  where  it  happens  to  be, 
will,  with  a  little  attention,  enable  one  to  find  it.    And 

1  It  will  be  remembered  that  the  planets  are  not  to  be  looked  for 
all  over  the  sky.  They  are  always  in  the  constellations  along  the 
zodiac. 


180  ASTRONOMY. 


when  once  found  it  may  be  followed  from  year  to  year, 
for  its  motion  is  so  slow  that  it  is  two  and  one-half  years 
in  passing  through  a  single  constellation. 

In  a  telescope  Saturn  is  unmistakable,  and  is  the  most 
beautiful  object  in  the  heavens  (Fig.  40).  The  rings, 
with  one  and  sometimes  two  of  the  satellites,  can  be 
seen  with  a  telescope  of  two  or  three  inches  of  aper- 
ture. The  other  satellites  and  the  marks  on  the  planet 
require  larger  instruments.  The  rings  are  widest  open 
in  1885  and  again  in  1899 ;  the  best  views  of  the  planet 
were  had  a  few  years  before  and  after  these  dates. 
The  years  1882-1885  were  exceptionally  favorable.  In 
1891-92  the  edge  of  the  ring  is  towards  the  earth. 
This  is  of  great  interest  to  astronomers  who  have  large 
telescopes,  but  for  owners  of  small  telescopes  Saturn 
about  this  time  possesses  little  interest. 

After  waiting  fifty  years,  a  new  satellite  of  Saturn 
was  discovered  in  1898  by  the  photographic  method. 
W.  H.  Pickering  found  on  his  plates  of  the  vicinity  of 
the  planet  a  faint  object  which  changed  its  position 
from  time  to  time  relatively  to  other  bodies.  Further 
observation  showed  this  to  be  a  ninth  satellite. 


URANUS.  181 


URANUS,     d 

Distance  from  the  Sun,  1,800,000,000  Miles.  Diameter,  32,000 
Miles.  Length  of  Year,  84  Years.  Length  of  Day  Unknown. 
Specific  Gravity,  1|.  Four  Satellites. 

174.  Position  and  Description  of  Uranus. — Uranus1  is 
the  third  in  order  from  the  sun,  and  the  smallest  in 
size,  of  the  outer  group  of  planets.     In  the  largest  tele- 
scopes no  markings  have  ever  been  certainly  seen  upon 
it :   it   shows   only  a  bright,  round   disk.     From  its 
great  size  and  its  position,  it  is  likely  that  it  resembles 
Jupiter,  and,  like  him,  may  not  have  cooled  off  yet. 
But  of  this  we  have  no  direct  evidence.     In  a  large 
telescope  Uranus  has  a  greenish  tinge. 

175.  Discovery  of  Uranus. — This  planet  was  discov- 
ered by  Sir  William  Herschel  in  1781.     Herschel  was 
a  German  music-teacher,  who  had  settled  in  England 
and  was  at  this  time  a  church  organist  in  the  city  of 
Bath.     Having  a   great   fondness   for   astronomy,  he 
made  a  number  of  telescopes  with  his  own  hands,  and 
became  a  diligent  amateur  observer.     At  the  time  of 
his  discovery  of  Uranus  he  was  almost  entirely  un- 
known as  an  astronomer.     But  this  discovery  at  once 
made  him  famous.     George  III.  made  him  his  private 
astronomer,  and  gave  him  a  pension  of  one  thousand 
dollars  a  year.     The  rest  of  his  life  was  devoted  en- 
tirely to  astronomy,  in  which  he  made  many  great  dis- 
coveries. 

When  first  discovered,  Uranus  was  supposed  to  be  a 


1  Unarms,  the  oldest  of  the  gods.     So  named  because  this  was  sup- 
posed to  he  the  most  distant  of  the  planets. 

16 


182  ASTRONOMY. 


tailless  comet ;  but  it  was  soon  found  to  be  a  planet. 
Its  discovery  awakened  the  greatest  enthusiasm  in  the 
scientific  world.  Not  even  a  new  satellite  had  been 
discovered  for  nearly  one  hundred  years,  and  all  of  the 
known  planets  had  been  known  from  the  very  earliest 
times.  It  was  found  that  Uranus  had  frequently  been 
seen  before,  but  had  always  been  taken  for  a  fixed 
star. 

176.  Satellites  of  Uranus. — Sir  "William  Herschel  an- 
nounced the  discovery  of  six  satellites  to  this  planet, 
but  it  is  now  generally  agreed  that  he  really  saw  only 
two,  and  that  he  was  mistaken  about  the  other  four. 
Two  others  were  afterwards  discovered,  and  it  is  cer- 
tain that  these  four  are  all  that  have  ever  been  seen, 
although  some  of  the  older  works  on  astronomy  still 
credit  Uranus  with  six.     The  last  two  discovered  are 
extremely  faint;  very  few  telescopes  will  show  them 
at  all. 

The  most  remarkable  fact  about  these  satellites  is, 
that,  unlike  every  body  in  the  solar  system  that  we 
have  yet  considered,  they  do  not  revolve  in  their  orbits 
from  west  to  east.  They  revolve  around  Uranus  almost 
from  north  to  south,1  and  what  little  motion  they  have 
in  the  other  direction  is  towards  the  west,  and  therefore 
retrograde. 

177.  Observations  of  Uranus. — When  nearest  to  the 
earth,  Uranus  is  just  visible  to  the  naked  eye,  and  looks 
exactly  like  a  very  faint  star.     Unless,  therefore,  one 
knows  just  where  it  is,  Uranus  cannot  be  distinguished 
by  the  naked  eye  from  the  stars.     Its  position  is  best 

1  That  is,  in  the  direction  which  is  to  us  north  and  south.  It  may 
be  that  Uranus  rotates  in  the  same  direction,  and  that  his  moons  there- 
fore revolve  about  him  nearly  parallel  to  his  equator. 


NEPTUNE.  183 


found  from  the  Nautical  Almanac,  where  its  right 
ascension  and  declination  (Art.  31)  are  given  for  each 
day  in  the  year.  These  will  show  its  position  on  a 
good  celestial  globe  or  map,  from  which  it  may  he 
found.1  But  it  is  scarcely  worth  finding,  for  to  the 
naked  eye,  or  in  an  ordinary  telescope,  it  possesses 
no  great  interest. 

NEPTUNE,    tj; 

Distance  from  the  Sun,  2,800,000,000  Miles.  Diameter,  35,000 
Miles.  Length  of  Year,  165  Years.  Length  of  Day  Unknown. 
Specific  Gravity,  1J.  One  Satellite. 

178.  Position  and  Description. — Neptune2  is  the  outer- 
most planet  of  the  solar  system.     It  is  rather  larger 
than  Uranus,  and  it  is  not  unlikely  that  it  resembles 
him- and  the  others  of  the  group  of  great  planets.     Of 
its  condition  nothing  can  be  determined  with  any  cer- 
tainty.    The  largest  telescopes   show  nothing  but  a 
small  bright  disk.     No  markings  can  be  seen  upon 
it,  and  therefore  the  period  of  its  rotation  cannot  be 
determined. 

179.  Discovery  of  Neptune. — After  Uranus  had  been 
discovered  and  watched  for  a  number  of  years,  it  was 
found  that  its  motion  was  not  quite  what  it  should 
be  if  acted  upon  solely  by  the  attraction  of  the  sun 
and  the  known  planets.     It  is  true  that  this  deviation 
was  very  slight:    it  amounted  altogether  only  to  2', 

1  When  a  telescope  is  properly  mounted,  it  may  be  pointed  at  once 
to  any  right  ascension  and  declination,  and  a  planet  or  star,  even  if 
invisible  to  the  naked  eye,  can  thus  be  found. 

2  Neptune,  god  of  the  sea,  son  of  Saturn,  and  brother  of  Jupiter- 
The  planet's  sign  is  the  trident  of  the  god. 


184  ASTRONOMY. 

or  one-sixteenth  of  the  apparent  diameter  of  the  moon. 
If  one  star  were  where  astronomers  had  -  calculated 
that  Uranus  ought  to  be,  and  another  were  where  it 
really  was,  the  two  would  seem  to  the  naked  eye  to 
be  one.  But  this  was  a  distance  entirely  too  great 
to  be  overlooked  in  astronomy.  So  it  began  to  be 
suspected  that  there  was  still  another  planet  outside 
of  the  orbit  of  Uranus,  which  by  its  attraction  was 
causing  this  deviation.  Two  young  mathematicians, 
Le  Yerrier.1  of  France,  and  Adams,2  a  student  at 
Cambridge  University,  England,  each  without  any 
knowledge  of  the  other,  attacked  the  problem.  This 
was  to  determine  whether  this  deviation  of  Uranus 
was  caused  by  the  attraction  of  an  unknown  planet, 
and,  if  so,  where  that  planet  must  be.  The  problem 
was  one  of  the  greatest  difficulty.  Adams  solved  the 
problem  first,  and  determined  very  nearly  the  true 
position  of  the  unknown  planet.  But  he  failed  to 
publish  his  results,  and,  although  he  sent  them  to  the 
Astronomer  Eoyal  of  England,  they  were  not  thought 
to  be  of  sufficient  importance  to  justify  a  search  for 
the  planet  with  a  telescope.  The  next  year,  1846,  Le 
Yerrier  reached  a  result,  which  he  published.  This 
was  found  to  agree  closely  with  that  of  Adams,  and 
search  for  the  planet  was  at  once  begun  at  Cam- 
bridge at  Adams's  suggestion.  But,  while  this  search 
was  going  on,  Le  Yerrier,  who,  like  Adams,  had  no 
telescope  at  his  command,  wrote  to  Dr.  Galle,3  of  Ber- 
lin, asking  him  to  point  his  telescope  to  a  certain  spot 

1  See  foot-note  on  page  65. 
*  John  Couch  Adams,  born  1819. 

8  Dr.  J.  G.  Galle  (Gal'eh),  1812-    ,  then  assistant  at  the  Berlin 
Observatory,  now  director  of  the  Observatory  at  Breslau,  Prussia. 


NEPTUNE.  185 


in  the  heavens,  and  to  look  for  the  new  planet.  Dr. 
Galle  did  so,  and  found  the  planet  within  less  than  one 
degree  of  the  place  designated. 

This  most  remarkable  discovery  in  all  the  history  of 
astronomy  excited  even  greater  enthusiasm  than  the 
discovery  of  Uranus.  It  made  Le  Verrier  probably 
the  foremost  and  most  famous  astronomer  of  the  world, 
a  place  which  he  held  for  the  rest  of  his  life.  In  this 
wonderful  discovery,  Adams,  prevented  by  no  fault  of 
his  own  from  being  the  chief  instrument  in  it,  has  re- 
ceived almost  equal  credit  with  Le  Verrier ;  and  their 
names  are  usually  coupled  together  in  the  story  of  the 
discovery  of  the  planet. 

180.  Neptune's  Satellite. — Only  one  satellite  has  been 
discovered  revolving  about  Neptune.     It  is  about  as 
faint  as  the  two  smaller  satellites  of  Uranus,  and  can 
be  seen  only  in  a  large  telescope.     Its  motion  is  still 
more  retrograde  than  the  motions  of  Uranus's  satellites. 
It  revolves  about  its  planet  from  east  to  west. 

181.  Observation  of  Neptune. — Neptune  is  never  visible 
to  the  naked  eye.     It  can  be  found  only  by  having  a 
telescope   properly  mounted,  and  pointing  it  to   the 
place  of  the  planet  in  the  heavens  as  given  in  the 
Nautical  Almanac.     In  an  ordinary  telescope  it  pos- 
sesses no  interest,  and  very  little  in  a  large  one. 

182.  The  View  of  the  Heavens  from  Neptune. — From 
Neptune  the  sun's  apparent  diameter  would  be  only 
about  one-thirtieth  of  his  apparent  diameter  to  us,  01 
about  the  same  as  that  of  Venus  when  she  is  nearest 
to  us.     Yet  his  light  would  be  more  than  one  hundred 
times  as  great  as  that  of  our  full  moon,  insignificant  as 
that  would  be  compared  with  the  light  which  we  re- 
ceive from  the  sun  (Art.  134).     Owing  to  the  smalinesb 


186  ASTRONOMY. 


and  brilliancy  of  the  sun,  it  is  not  likely  that  he  would 
shew  any  disk  at  all  to  an  observer  on  Neptune,  but 
would  be  only  an  exceedingly  brilliant  star.  Uranus 
and  Saturn,  and  possibly  Jupiter  at  times,  are  the  only 
planets  which  could  be  seen  by  the  naked  eye  from 
Neptune.  All  the  planets  within  Jupiter's  orbit  would 
be  too  close  to  the  sun  to  be  seen.  Although  seemingly 
so  far  out  towards  the  stars,  they  would  not  be  appre- 
ciably brighter  at  Neptune  than  upon  the  earth.  For, 
as  we  shall  presently  learn,  the  nearest  star  is  so  far 
away  that  the  distance  to  Neptune,  enormous  as  it  is, 
becomes  as  nothing  beside  it. 

Are  the  Planets  Inhabited? 

183.  This  is  a  question  of  great  popular  interest,  and 
one  that  is  often  asked.  But  it  is  one  to  which  astrono- 
mers pay  very  little  attention,  because  of  the  impos- 
sibility of  finding  any  satisfactory  answer  to  it.  The 
only  heavenly  body  near  enough  to  us  to  allow  us  to 
form  an  intelligent  opinion  about  its  being  inhabited 
is  the  moon.  And  there  the  absence  of  air  and  water, 
and  of  any  changes  such  as  would  be  caused  by  our 
seasons,  makes  us  certain  that  no  life  such  as  we  know 
anything  about  exists.  So  far  as  we  can  tell,  Mars 
most  resembles  the  earth,  and  it  has  often  been  sup- 
posed that  it  may  be  inhabited.  But  upon  Mars  one 
would  receive  less  than  half  as  much  heat  from  the 
sun  as  he  gets  upon  the  earth.  This,  unless  modified 
by  other  circumstances,  would  reduce  the  temperature 
of  Mars's  surface  far  below  zero, — a  condition  which 
of  itself  would,  as  it  seems  to  us,  make  it  impossible 
for  life  to  exist  there.  As  has  been  said,  there  is  a 


ARE  THE  PLANETS  INHABITED  f  187 

strong  probability  that  Jupiter  and  Saturn,  and  per- 
haps Uranus  and  Neptune,  are  still  intensely  hot, 
and,  if  so,  incapable  of  sustaining  life.  If  they  have 
cold,  solid  crusts  like  that  of  the  earth,  the  argument 
against  the  habitability  of  Mars  would  apply  to  them 
with  increased  force.  But  Prof.  Tyndall l  has  pointed 
out  that  if  the  atmospheres  of  these  distant  planets 
were  composed  in  part  of  certain  vapors  known  to 
us,  they  would  admit  the  sun's  heat  freely,  but  would 
prevent  it  from  passing  out  again  :  just  as  window- 
glass  allows  the  sun's  heat  to  pass  through  it  into  a 
room,  but  allows  very  little  of  the  heat  of  the  room 
to  pass  out.  Such  an  atmosphere  would  store  up 
the  sun's  heat,  and  might  make  the  distant  planets 
inhabitable. 

Of  the  inferior  planets,  the  one  that  seems  to  us  most 
likely  to  be  inhabited  is  Venus.  As  we  never  see 
Venus's  surface  through  her  dense  atmosphere,  we  do 
not  know  how  much  resemblance  she  bears  to  the 
earth.  But,  as  she  receives  twice  as  much  heat  from 
the  sun  as  the  earth  gets,  it  seems  to  us  scarcely  pos- 
sible for  life  to  exist  there.  Still,  it  may  be  that  Venus 
has  an  atmosphere  so  dense  as  to  protect  her  surface 
from  the  intense  heat  of  the  sun's  rays. 

The  question  may  be  summed  up  by  saying  that  if 
the  earth  as  now  constituted  were  suddenly  put  into 
the  position  of  any  one  of  the  other  planets,  it  seems 
certain  that  all  life  upon  it  would  be  speedily  destroyed. 
As  to  whether  varieties  of  life  adapted  to  the  different 
planets  exist  there,  or  whether  these  planets  have  con- 

1  John  Tyndall,  born  1820,  Professor  of  Natural  Philosophy  in  the 
Royal  Institution  of  Great  Britain.  He  is  one  of  the  greatest  of 
living  scientists. 


188  ASTRONOMY. 


ditions  and  surroundings  unknown  to  the  earth  which 
adapt  them  to  such  life  as  exists  here,  we  do  not  know, 
and  in  all  probability  shall  never  find  out.  "  Here  we 
may  give  free  rein  to  our  imagination,  with  the  moral 
certainty  that  science  will  supply  nothing  tending  either 
to  prove  or  to  disprove  any  of  its  fancies." 


COMETS  AND  METEORS.  189 


CHAPTER  VIII 

COMETS   AND   METEORS. 

have  so  far  considered,  in  the  solar  system,  sun, 
planets,  and  moons.  There  yet  remain  to  be  men- 
tioned two  other  classes  of  bodies,  comets  and  meteors. 
We  will  first  treat  of  these  separately,  and  then  show 
an  interesting  connection  between  the  two. 

Comets. 

184.  General  Appearance. — A  comet  is  a  body  which, 
when  visible  to  the  naked  eye,  usually  presents  the 
appearance  of  a  star  with  a  tail  extending  out  to  one 


FM.  43.— TELESCOPIC  COMET  FIG.  44.— TELESCOPIC  COMET 

WITHOUT  A  NUCLEUS.  WITH  A  NUCLEUS. 

side.  This  tail  nearly  always  points  away  from  the  sun. 
The  smaller  comets,  such  as  can  be  seen  only  with  a 
telescope,  frequently  have  no  tail  at  all,  being  simply 


190  ASTRONOMY. 


round  masses  of  hazy  light,  either  uniformly  bright,  or 
with  a  brighter  spot  near  the  centre.  Figs.  43  and  44 
show  two  telescopic  comets,  one  with  and  the  other 
without  a  central  condensation. 

185.  Parts  of  Comets. — Comets  are  usually  composed 
of  three  parts,  the  nucleus,  the  coma,  and  the  tail.  The 
nucleus  is  the  star-like  head  in  which  a  large  portion 
of  the  light  is  concentrated.  It  is  often  bright  enough 
to  be  s^en  in  the  daytime.  In  1843  and  1858,  in  recent 
times,  and  in  mary  cases  mentioned  in  history,  comets 
have  been  distinctly  noticed  in  the  midst  of  the  glare 
of  sunlight.  Though  so  bright,  it  is  known  that  they 
have  very  little  substance.  While  giving  sometimes 
more  light  than  the  planet  Jupiter,  they  are  probably 
not  one-millionth  part  as  heavy.  We  know  this  from 
the  slight  effect  they  have  on  the  planets  when  they 
approach  them.  A  heavy  body  would  attract  a  planet 
out  of  its  path  and  change  its  orbit  about  the  sun. 
Nothing  of  the  kind  has  ever  been  noticed.  It  is  sup- 
posed fhat  on  one  occasion  the  earth  passed  through 
the  tail  of  a  comet.  In  1770  a  comet  was  discovered, 
which,  in  its  approach  to  the  sun,  passed  very  close  to 
Jupiter,  and  remained  in  its  neighborhood  for  several 
months.  But  it  did  not  seem  to  have  any  effect  what- 
ever on  Jupiter  or  his  satellites,  while  the  planet's  at- 
traction changed  the  comet's  orbit  completely. 

The  coma1  is  the  envelope  immediately  surrounding 
the  nucleus.  It  usually  shades  off  from  it,  so  that  no 
distinct  line  of  separation  can  be  seen.  Frequently  it 
is  composed  of  a  series  of  circular  bands  of  light,  as 
in  the  drawing  on  the  following  page,  made  from  a 


1  Latin  for  hair. 


COMETS  AND  METEORS.  191 

telescopic  view,  of  Coggia's  comet.     The  nucleus  and 
coma  together  make  up  the  head. 

The  tail  stretches  out  from  the  coma,  growing  fainter 


FIG.  45. — COOGIA'S  COMET,  1874. 


till  entirely  lost.  It  usually  broadens  as  it  recedes  from 
the  head.  It  also  is  extremely  thin  and  rare.  Even 
very  faint  stars  can  be  seen  through  it.  In  the  case  of 


192 


ASTRONOMY. 


Fi«.  46.— DONATI'S  COMET,  1858. 


COMETS  AND  METEORS.  193 

the  faint  comets  the  tail  is  often  not  distinguishable  at 
all,  while  in  many  of  the  brighter  ones  mentioned  in 
history  it  stretched  from  the  horizon  to  the  zenith. 
Thus,  we  have  an  account  of  a  comet  in  the  year  134 
B.C.  that  "  lasted  seventy  days ;  the  heavens  appeared 
all  on  fire ;  the  comet  occupied  a  fourth  part  of  the  sky, 
and  its  brilliancy  was  superior  to  that  of  the  sun ;  it  took 
four  hours  to  rise  and  four  hours  to  set."  Fig.  46 
shows  the  appearance  of  the  tail  of  Donati's  comet 
of  1858.  What  the  faint  comets  lack  in  brilliancy 
and  extent  of  tail  they  make  up  in  number.  Six  or 
more  telescopic  comets  are  frequently  discovered  in 
one  year,  while  but  few  great  naked-eye  comets  are 
seen  in  a  lifetime.  Since  the  Christian  era  about  five 
hundred  comets  have  been  recorded  as  visible  to  the 
unaided  eye,  while  the  number  of  telescopic  comets 
seems  to  justify  the  saying  of  Kepler,  that  celestial 
space  is  as  full  of  comets  as  the  sea  is  of  fish. 

186.  Orbits  of  Comets. — Comets  sometimes  move  in 
ellipses  and  sometimes  in  parabolas  and  hyperbolas.1 
"When  the  orbit  is  either  of  the  latter  curves,  they 
approach  the  sun  from  outside  space,  we  know  not 
whence,  swing  around  it,  and  go  away  never  to  return. 
When  it  is  an  ellipse,  they  move  around  the  sun  like 
the  planets,  returning  again  and  again  to  the  same 
point.  The  latter  kind  are  called  periodic  comets,  be- 
cause they  have  a  regular  period  of  revolution.  Their 
reappearance  can  be  expected,  and  the  exact  point  in 
the  heavens  where  they  may  be  seen  at  a  given  time 
accurately  calculated.  The  ellipses  which  the  comets 

1  Ellipses  and  parabolas  have  been  explained  on  page  33.  A  hy- 
perbola is  a  curve  resembling  in  general  appearance  a  parabola,  Jit 
is  not  closed  up  like  an  ellipse. 


194 


ASTRONOMY 


describe  are,  however,  much  flatter  than  those  of  the 
planets.  Fig.  47  shows  the  orbit  of  Halley's1  comet. 
Its  point  farthest  from  the  sun  is  outside  of  Neptune's 
orbit,  and  its  nearest 
about  the  distance  of  the 
earth.  Such  a  comet, 
when  far  from  the  sun, 
will  move  very  slowly, 
but  as  it  approaches,  its 
velocity,  like  that  of  a 
falling  body,  will  in- 
crease. It  will  swing 
around  the  sun  with  im- 
mense rapidity,  and  fly 
off,  moving  continually 
more  and  more  slowly. 

Many  of  the  comets 
have  doubtless  been  se- 
cured to  the  solar  system 
as  permanent  members 
by  the  attraction  of  the 
planets.  If  a  comet, 
coming  in  from  outside 

FIG.  47.— ORBIT  OF  HALLEY'S  COMET. 

space  in  a  parabola,  were 

to  pass  in  front  of  a  planet,  the  planet's  attraction 
might  diminish  its  velocity,  and  change  its  orbit  to  an 
ellipse.  The  comet  of  1770  thus  came  in  from  with- 
out, but  Jupiter  changed  its  orbit  to  a  small  ellipse, 
with  a  period  of  five  and  one-half  yeare.  It  then  per- 
formed two  revolutions  around  the  sun,  when  it  again 

1  Halley,  a  friend  of  Newton,  lived  in  England  1656  to  1742. 
Comets  are  sometimes  named  after  their  discoverers,  and  sometime* 
after  the  astronomers  who  calculate  their  orbits. 


COMETS  AND  METEORS.  195 

approached  the  great  planet,  and  had  its  orbit  affected 
so  that  it  has  never  since  been  seen.  A  number  of 
comets  revolve  about  the  sun  having  their  aphelia1  at 
about  Jupiter's  distance,  and  it  is  supposed  that  these 
have  all  in  past  times  been  delayed  by  Jupiter,  and  so 
changed  into  permanent  members  of  our  system. 

187.  Growth  of  Comets. — When  a  comet  is  first  de- 
tected by  a  telescope  as  it  approaches  the  sun,  it  usually 
appears  as  a  round  mass  of  hazy  light  of  uniform 
brightness.  Presently  an  increase  of  brightness  shows 
itself  in  one  point,  which  gradually  grows  into  a 
nucleus.  As  it  still  draws  nearer  to  the  sun,  an  arch 
is  seen  partly  to  envelop  the  nucleus  on  the  side 
next  the  sun;  this  grows  longer  and  brighter,  and 
finally  the  tail  itself  begins  to  grow  away  from  the 
sun,  reaching  out  farther  and  farther  as  the  comet 
nears  the  sun,  till  at  perihelion2  it  is  longest  and 
brightest.  Sometimes  a  number  of  envelopes  surround 
the  nucleus,  the  whole  presenting  the  appearance  of 
a  fountain  shooting  out  towards  the  sun  and  then  fall- 
ing away  from  it.  This  appearance  is  shown  in  the 
drawing  of  Coggia's  comet,  and  also  in  those  of  the 
comet  of  1861.  The  rapidity  with  which  the  tails  of 
comets  grow  is  wonderful;  an  increase  of  35,000,000 
miles  in  a  single  day  is  recorded  in  one  case.  As  the 
comet  recedes  from  the  sun  it  goes  through  its  changes 
in  reverse  order,  apparently  drawing  in  its  tail,  and, 
finally,  its  envelopes,  and  returning  to  its  condition  of 
uninteresting  uniformity.  It  has  its  brief  day  of  light 
and  activity,  to  be  followed  by  a  long  night  of  dark- 
ness and  rest. 

1  Points  farthest  from  the  sun.  s  Point  nearest  the  sun. 


196  ASTRONOMY. 


188.  What  are  Comets  ? — This  is  a  question  to  which 
it  is  difficult  to  give  a  complete  answer.  The  spectro- 
scope seems  to  indicate  that  they  are  partly  solid  and 


FIG.  48.— COMET  OF  1861. 


partly  gaseous.  The  nucleus  is  probably  solid,  and 
shines  by  reflecting  the  light  from  the  sun.  The  coma 
and  tail  are  gaseous,  the  former  giving  out  light  of  its 


COMETS  AND  METEORS.  197 

own.  This  light  is  due  largely  to  glowing  carbon 
vapor,  and  resembles  somewhat  the  flame  of  ordi- 
nary house-gas.  It  is  a  property  of  gases  to  expand 
indefinitely,  unless  held  in  by  a  central  attraction; 
also  it  is  known  that  in  one  case  at  least  a  comet  van- 
ished from  sight,  and  a  shower  of  solid  meteors  took 
its  place.  We  may,  then,  think  of  a  comet  as  a  solid 
nucleus  or  a  collection  of  solid  particles  surrounded  by 
a  dense  gaseous  atmosphere  in  a  state  of  great  activity. 
This  activity  shows  itself  in  the  violent  and  rapid 
changes  which  are  often  noticed  in  the  heads  of  comets. 
Sometimes  a  mass  is  thrown  off  from  the  nucleus  and 
forms  a  separate  body,  which  afterwards  disappears. 
Sometimes  there  are  jets  seen  extending  from  the  nu- 
cleus towards  the  sun  and  on  either  side,  which  usually 
curve  backward  into  the  tail.  The  sun  seems  to  have 
a  repulsive  instead  of  an  attractive  force  upon  it,  and 
the  substance  shot  out  into  the  tail  represents  so  much 
waste  matter  to  the  comet.  The  tail  is  probably  in  the 
form  of  a  hollow  tube,  as  its  edges  often  seem  brighter 
than  its  centre.  It  is  kept  up  by  a  constant  flow  of 
particles  from  the  head.  The  enormous  velocity  of  the 
head  around  the  sun  makes  it  improbable  that  the  tail 
is  rigidly  attached  to  it,  for  if  so  it  would  be  cast  away 
by  the  great  centrifugal  force  of  the  outer  end.  It  is 
generally  curved  backward,  thus  showing  that  the  par- 
ticles as  they  move  out  retain  only  the  slower  motion 
of  the  inner  parts,  and  are  therefore  left  behind.  The 
bright  comet  of  1881  showed  a  great  many  of  these 
changes  and  was  carefully  watched. 

189.  Danger  from  Comets. — The  ancients  looked  upon 
comets  as  the  forerunners  of  war,  pestilence,  the  death 
of  kings,  and  all  things  evil.  Their  writings  hence  give 

17* 


198  ASTRONOMY. 


numerous  and  detailed  descriptions  of  many  of  them, 
and  astronomy  is  thereby  the  gainer.  The  real  danger 
from  a  comet  is  that  the  large  nucleus  of  one  might 
strike  the  earth.  This  would  produce  such  a  blinding 


mmm^mmmmmmmmmmm*mmi^^^m**fmm*mmmmmmm^m^a*^a^^mmnmm*m^^B*Bmmmimm^im*i 

FIG.  49.— COMET  OF  1882,  as  seen  on  the  morning  of  Sept.  30.     A.  Naked-eye  view.    B. 


Telescopic  appearance  of  head. 

light  and  intense  heat  that  all  life  on  that  side  of  the 
earth  would  be  immediately  destroyed.  Most  nuclei 
are,  however,  too  small  to  produce  such  serious  effects, 
and  the  chances  of  striking  are  so  slight  that  we  may 
as  well  dismiss  all  thought  of  its  happening. 

190.  Remarkable  Comets. — Halley's  comet  was  remark- 
able as  being  the  first  whose  period  of  revolution  was 
calculated  or  even  suspected.  After  Newton  had  dis- 
covered the  cause  of  the  motion  of  the  planets  around 


COMETS  AND  METEORS.  199 

the  sun,  he  predicted  that  comets  would  be  found  to 
obey  the  same  laws,  and  hence  that  their  regular  re- 
turn might  be  expected.  This  set  Halley  to  work  to 
searching  among  the  old  records  to  find  comets  sepa- 
rated by  uniform  distances  of  time  and  whose  orbits 
agreed.  The  comet  of  1682  had  just  been  an  object  of 
interest,  and  his  search  showed  that  in  1531  and  1607 
similar  ones  had  appeared  :  he  therefore  felt  justified  in 
stating  that  they  were  successive  returns  of  the  same 
object,  and  that  it  would  reappear  about  the  early  part 
of  1759.  The  event  corresponded  to  the  prediction. 
On  the  night  of  Christmas-day,  1758,  it  was  first  seen, 
and  it  reached  its  perihelion  passage  on  March  12, 
1759.  It  returned  again  in  1835,  and  its  next  return 
will  probably  be  in  1910. 

191.  Micke's l  comet  has  probably  been  studied  more 
than  any  other.  It  was  discovered  at  various  times 
by  different  observers,  among  others  by  Caroline  Her- 
schel,2  though  it  was  never  suspected  that  it  was  the 
same  object.  Encke,  finding  that  its  orbit  was  not  a 
parabola,  went  into  an  elaborate  investigation,  and 
found  that  it  revolved  in  an  ellipse  with  the  very  short 
period  of  three  and  one-half  years.  Then,  counting 
back,  he  found  many  records  of  its  previous  discov- 
ery. It  has  been  watched  several  times  since. 

This  comet  is  of  special  interest  to  astronomers 
from  the  fact  that  its  time  of  revolution  is  not  uni- 
form. It  arrives  at  its  perihelion  about  two  and  one- 
half  hours  before  the  calculated  time,  so  that  its  peri- 

1Enk/eh,  a  German,  1791-1865. 

2  Caroline  Herschel,  the  sister  of  Sir  William  Herschel,  and  his 
faithful  assistant  in  much  of  his  astronomical  work. 


200  ASTRONOMY. 


odic  time  has  diminished  two  days  since  it  was  first 
discovered.  This  indicates  that  it  must  be  continually 
approaching  the  sun,  as  a  small  orbit  and  short  periodic 
time  always  go  together.  The  only  plausible  explana- 
tion of  this  fact  is  that  the 
comet,  being  very  light,  is  re- 
sisted in  its  motion  by  the 
ether  which  is  supposed  to 
fill  all  space.  This  resist- 
ance would  tend  to  decrease 
the  velocity  and  centrifugal 
force,  and  hence  the  comet 
would  gradually  fall  towards 

the  fun-     Jt  is  a  sma]1  tele- 
scopic  comet,  usually,  though 

not  always,  seen  without  a  tail.     Fig.  50  represents  its 
appearance  at  one  of  its  returns. 

192.  Biela's l  comet  is  remarkable  for  other  reasons. 
Like  Encke's,  it  was  a  small  telescopic  comet,  and  it  had 
a  period  of  about  six  and  one-half  years.  Gibers2  had 
called  attention  to  the  fact  that  in  1832  it  would  pass 
within  20,000  miles  of  the  earth's  orbit,  though,  as  the 
earth  would  not  reach  the  same  point  till  a  month  later, 
no  danger  was  apprehended  by  astronomers.  Many 
other  people,  however,  looked  forward  to  the  time  with 
considerable  anxiety.  The  comet  came  punctually  as 
predicted,  but  no  barm  resulted  therefrom.  In  1846 
it  again  returned  very  close  to  the  earth,  and  was  care- 
fully studied.  Much  to  the  surprise  of  astronomers, 
while  their  telescopes  were  pointed  at  it,  it  began  to 
divide  into  two  comets,  which  gradually  receded  from 

1  Bie'la,  a  German,  1782-1856. 
8  Ol'bers,  a  German,  1758-1840. 


COMETS  AND  METEORS.  201 

each  other  as  long  as  they  were  visible.  When  it  came 
back  in  1852  the  division  had  increased,  and  measured 
one  and  one-quarter  millions  of  miles.  At  the  return 
of  1859  it  would  not  be  in  a  good  position  for  observa- 
tion, but  in  1866  it  was  expected  with  great  interest, 
in  order  to  notice  what  further  changes  might  have 
taken  place;  but  Biela's  comet  has  never  since  been 
seen.  No  satisfactory  explanation  of  its  division  and 


FIG.  51.— BIELA'S  COMET,  1846. 

subsequent  disappearance  has  ever  been  offered.  Some 
other  interesting  facts  in  connection  with  this  comet 
will  be  given  in  the  section  on  meteors.  The  drawing 
represents  its  appearance  just  after  the  separation. 

193.  Comet  6,1  1881. — This,  the  brightest  comet  since 
the  spectroscope  has  been  perfected,  was  observed  more 
carefully  than  any  other.  The  tail  was  nearly  40°  long, 
and  the  head  was  as  bright  as,  and  larger  than,  a  first- 

1  So  many  comets  are  now  discovered  that  all  of  one  year  are  named 
by  the  letters  of  the  alphabet.  This  was  the  second  one  of  1881 


202  ASTRONOMY. 


magnitude  star.  Most  of  the  phenomena  described  in 
paragraphs  187  and  188  were  noticed  in  this  comet. 
At  one  time  a  separation  in  the  nucleus  led  astronomers 
to  think  that  it  would  follow  the  example  of  Biela's 
comet  (see  Fig.  51);  but  by  the  following  night  the 
smaller  part  had  disappeared.  Very  frequent  changes 
were  observed  by  the  telescope,  in  the  shape  of  the  jets 
and  envelopes  around  the  nucleus.  Many  drawings 
were  made  of  it,  it  was  carefully  studied  by  means  of 
the  spectroscope,  and  (for  the  first  time  with  comets) 
a  photograph  was  taken  of  it. 

Cornet  6,  1882. — This  comet  came  very  unexpect- 
edly, and  was  seen  about  September  17  near  the  sun 
in  broad  daylight.  It  then  passed  to  the  east  of  the 
sun,  and  appeared  as  a  brilliant  object  to  the  naked 
eye  for  two  months.  In  the  telescope  its  nucleus  was 
observed  to  undergo  rapid  changes,  dividing  into  two 
or  more  parts,  which  afterwards  seemed  to  come  to- 
gether again.  An  extension  of  the  tail  several  degrees 
from  the  nucleus  towards  the  sun  was  visible  at  one 
time.  The  tail  as  seen  by  the  eye  was  about  15°  long 
and  2°  broad. 

When  its  orbit  was  calculated  it  was  found  to  agree 
quite  closely  with  that  of  comets  which  had  appeared 
in  1843  and  1880,  but  a  further  computation  showed 
that  it  moved  in  a  vast  ellipse,  taking  (according  to 
one  computer)  about  800  years  to  go  around.  This 
would  carry  it  to  a  distance  from  the  sun  about  five 
times  as  far  as  is  Neptune.  At  its  perihelion  it  passed 
within  half  a  million  miles  from  the  solar  surface,  while 
at  aphelion  it  will  be  something  like  16,000  times  as 
far  away.  The  breadth  of  the  ellipse  would  be  about 
one-sixtieth  its  length. 


COMETS  AND  METEORS.  203 

"While  it  is  not  the  same  comet  as  those  of  1843  and 
1880,  yet  it  is  hardly  likely  that  the  agreement  of  their 
orbits  is  accidental.  Some'  unknown  connection  prob- 
ably exists  among  them. 

The  Lexell-Brooks  Comet. — A  recent  comet  with  an 
interesting  history  is  this  one.  It  first  appeared  in 
1770,  and  an  orbit  of  five  and  a  half  years  was  com- 
puted for  it  by  Lexell.  It  was  never  seen  again  till 
1889.  In  the  mean  time  it  seems  probable  it  had  had 
a  devious  course.  Jupiter,  which  had  first  brought  it 
into  the  solar  system  by  changing  its  orbit  from  a  par- 
abola to  a  small  ellipse,  changed  it  again  to  a  large 
ellipse,  and  now,  after  more  than  a  century,  has  brought 
it  again  into  visibility.  It  is  not  unlikely  that  it  will 
again  approach  Jupiter,  when  some  other  change,  as 
yet  unknown,  may  be  expected. 

194.  How  to  Look  for  Comets. — From  what  has  been 
said  we  iriay  infer  that  new  and  interesting  comets  may 
be  expected  at  any  time  and  in  any  part  of  the  heavens. 
The  number  of  those  whose  return  can  be  predicted  is 
very  small  as  compared  with  the  number  of  new  ones 
which  may  be  discovered.  Prizes  of  medals  or  sums 
of  money  have  been  offered  at  various  times  for  the 
discovery  of  comets,  and  this  and  the  honor  connected 
with  it  have  led  many  to  search  for  them  with  great 
perseverance.  The  work  does  not  need  large  telescopes, 
nor  does  it  require  any  skill  which  an  ordinary  person 
with  a  good  eye  cannot  acquire.  The  instrument  used 
should  have  a  low  power,  such  a  one  as  would  magnify 
about  twenty-five  times,  and  it  should  have  a  large 
field  of  view.  The  comet  is  detected  partly  by  its  ap- 
pearance and  partly  by  its  change  of  place  among  the 
stars.  Judging  by  appearance  alone,  a  nebula  or  cluster 


204  ASTRONOMY. 


might  be  mistaken  for  a  comet.  But  a  little  watch  will 
show  whether  there  is  any  change  of  position  with  refer- 
ence to  the  neighboring  stars.  If  a  discovery  is  made, 
it  should  be  immediately  telegraphed  to  some  observa- 
tory. Observers  should  be  supplied  with  a  good  cata- 
logue of  clusters  and  nebulae. 


Meteors. 

195.  General  Remarks. — Every  one  is  familiar  with 
"  shooting-stars."     This  name  describes  their  general 
appearance,  but  in  reality  they  are  very  different  from 
stars.     The  stars  are  all  at  an  immense  distance  from 
us,  while  meteors  are  in  our  atmosphere.      Stars  are 
very  large  bodies,  while  meteors  are  quite  small.     No 
instance  of  visible  motion   in   a   star  has  ever  been 
noticed,  nor  is   it  likely  ever  to  be.      When  we  use 
the  name  "  shooting-star,"  we  must  remember  the  great 
difference  which   exists  between   them   and  the  real 
stars. 

Upon  almost  any  clear,  moonless  night  we  can  see 
meteors.  It  has  been  calculated  that  the  average 
number  visible  is  about  five  an  hour.  Sometimes  it 
vastly  exceeds  this.  As  many  as  30,000  an  hour  is 
given  as  the  number  seen  on  several  occasions.  When 
a  watch  for  them  is  carefully  maintained,  it  is  noticed 
that  on  certain  nights  of  the  year  they  are  especially 
numerous.  About  the  10th  of  August  and  the  14th 
of  November  a  person  cannot  fail  to  notice  a  large 
number  during  any  clear  and  moonless  night. 

196.  What  are  Meteors? — Meteors  are  small,  cold. 


COMETS  AND  METEORS.  205 

solid  bodies,  sometimes  probably  merely  misty  clouds  of 
light  matter,  which  are  revolving  about  the  sun  entirely 
independently  of  the  earth.  Sometimes,  however,  as 
the  earth  is  moving  on  its  orbit  with  tremendous  ve- 
locity, it  approaches  some  of  these  little  bodies.  The 
earth's  atmosphere  probably  extends  a  hundred  or  more 
miles  in  all  directions  from  its  surface,  and  the  meteors 
strike  it  with  great  energy.  Now,  we  know  that  heat 
is  produced  when  a  nail  is  struck  with  a  hammer;  just 
so  when  a  meteor,  probably  with  a  great  velocity  of  its 
own,  comes  in  contact  with  the  atmosphere  moving  to 
meet  it  at  a  rate  of  66,000  miles  per  hour,  it  is  made 
so  hot  that  it  burns.  It  keeps  on  moving  until  entirely 
consumed,  and  thus  we  see  the  blazing  streak  across 
the  sky.  The  train  which  is  sometimes  seen  to  follow 
it  and  to  float  away  like  a  light  cloud  is  the  red-hot 
ashes,  which  gradually  and  imperceptibly  fall  to  the 
earth.  Sometimes  the  concussion  is  so  violent  as  to 
break  the  meteor  into  fragments,  and  we  hear  a  loud 
report  and  see  the  flying  masses  in  their  separate  tracks. 
If  the  body  be  very  large  and  bright,  and  seem  to  pass 
a  long  way  through  the  atmosphere,  and  to  approach 
very  near  the  earth,  it  is  called  a  fire-ball;  and  if  still 
larger,  so  as  not  to  be  consumed  by  the  time  it  reaches 
the  earth,  it  is  usually  called  an  aerolite.1  Many  aero- 
lites have  been  seen  to  fall  which  have  afterwards  been 
picked  up  and  examined.  They  show  the  indications 
of  intense  heat  in  their  glazed  surfaces,  which  have  evi- 
dently been  molten.  This  has  been  done  in  their  pas- 
sage through  the  atmosphere.  But,  besides  this,  when 
they  are  carefully  examined  with  a  microscope  their 


1  A'erolite,  a  stone  from  the  air. 
18 


206  ASTRONOMY. 


whole  interior  structure  is  found  to  be  crystalline :  this 
proves  that  at  some  time  they  must  have  been  either 
wholly  molten  or  gaseous.  They  are  usually  largely 
composed  of  iron,  but  often  have  a  number  of  other 
substances  in  connection  with  it,  though  they  contain 
no  elements  which  are  not  found  on  the  earth.  It  is 
interesting  to  know  that  the  matter  that  comes  in  to  us 
from  outside  space  contains  the  same  substances  which 
exist  here.  We  have  seen  that  the  sun  is  made  up 
largely  of  terrestrial  elements,  and  we  shall  find  the 
same  to  be  true  of  the  stars.  It  is  probable  that  on 
the  earth  we  have  specimens  of  nearly  every  element 
that  exists  in  the  universe. 

It  is  calculated  that  meteors  begin  to  burn  at  the 
height  of  about  seventy  miles  from  the  earth,  and  the 
smaller  and  more  inflammable  are  usually  consumed 
after  a  course  of  about  forty  miles  through  the  air. 
They  are  mostly  small,  usually  about  a  grain  in  weight, 
though  many  are  larger. 

197.  But  how  are  these  little  masses  arranged  in 
space?  It  is  known  that  they  are  not  uniformly  dis- 
tributed, but  that  they  are  collected  into  rings  about 
the  sun.  These  rings  are  not  circular,  but  are  elliptic, 
like  the  orbit  of  a  comet,  having  the  sun  in  a  focus. 
There  are  many  millions  of  meteors  in  every  ring,  and 
they  follow  one  another  in  this  ring  round  and  round 
the  sun.  They  are  in  reality  very  minute  planets  which 
have  been  crowded  together  and  are  performing  their 
revolution  around  the  sun  in  company,  every  one,  no 
doubt,  much  influenced  by  the  attractions  of  the  others. 
The  meteors  are  not  always  arranged  in  the  rings 
uniformly,  but  are  collected  in  groups,  with  compara- 
tively barren  spaces  between  them. 


COMETS  AND  METEORS.  207 

198.  November  and  August  Meteors. — While  the  me- 
teors themselves  revolve  about  the  sun,  the  ring,  con- 
sidered as  a  whole,  retains  the  same  position  all  the 
time.  Hence  if  the  earth  passes  through  the  ring  in 
one  of  its  revolutions,  it  will  pass  through  it  on  each 
succeeding  revolution  at  the  same  time  of  year.  Every 
year,  accordingly,  when  the  earth  reaches  the  point  of 
its  orbit  which  intersects  the  ring,  unusual  displays 
may  be  looked  for.  If  the  meteors  were  distributed 
uniformly  along  the  ring,  the  same  number  would  be 
seen  every  year;  but  if  there  were  one  main  bunch 
in  the  ring  and  the  remainder  of  the  meteors  were 
more  thinly  scattered  along,  the  display  would  be  most 
striking  when  the  earth  encountered  this  main  body. 
Since  the  meteors  have  a  regular  time  of  revolution 
about  the  sun,  we  might  expect  these  striking  dis- 
plays to  recur  at  regular  intervals,  when  the  main 
bunch  comes  around. 

Such  is  the  case.  Every  year  on  the  14th  of  No- 
vember, and  on  several  nights  on  both  sides  of  this 
date,  unusual  showers  of  meteors  are  observed ;  while 
at  intervals  of  about  thirty-three  years  the  display  is 
remarkably  brilliant.  One  of  these  occurred  on  No- 
vember 12,  1799,  when  Humboldt1  saw  it  in  South 
America,  and  described  it  as  follows  :  "  Thousands  of 
bodies  and  falling  stars  succeeded  each  other  during 
four  hours.  From  the  beginning  of  the  phenomenon 
there  was  not  a  space  in  the  firmament  equal  in  extent 
to  three  diameters  of  the  moon  which  was  not  filled 
every  instant  with  falling  stars."  On  November  13, 
1833,  the  most  brilliant  display  of  meteors  on  record 

1  Hum'bOlt.     A  great  German  scientist,  1769  to  1859. 


208  ASTRONOMY. 


occurred.  It  was  visible  all  over  North  America.  The 
whole  heavens  seemed  on  fire,  and  the  greatest  con- 
sternation prevailed  among  ignorant  people.  Again 
on  November  14,  1866,  the  shower  returned,  this  time 
visible  in  England.  It  was  observed  with  care,  and 
about  8000  were  counted  in  one  night  at  the  Green- 
wich Observatory.  As  the  meteor-bunch  occupied 
some  time  in  passing  the  point  of  contact  with  the 
orbit  of  the  earth,  the  display  appeared  for  several 
years  succeeding  this. 

The  explanation  of  these  regularly  returning  showers 
has  already  been  indicated.  Every  year,  on  the  14th 
of  November,  the  earth  in  the  course  of  its  journey 
around  the  sun  passes  through  this  meteoric  ring,  and 
we  have  the  yearly  display.  But  the  meteors  which 
constitute  the  ring  are  not  uniformly  distributed  along 
its  course.  There  is  one  main  collection  of  them 
and  probably  several  smaller  ones.  Moreover,  these 
meteors  are  themselves  revolving  about  the  sun,  com- 
pleting a  revolution  in  thirty-three  and  one-fourth 
years.  Hence  at  intervals  of  this  time  the  earth  en- 
counters this  main  swarm  and  ploughs  a  path  through 
it.  The  bodies  are  attracted  to  the  earth,  which  is  also 
advancing  to  meet  them,  and  the  collision  and  friction 
with  the  atmosphere  give  us  the  splendid  displays  of 
shooting-stars.  Fig.  52  shows  how  the  earth's  orbit 
comes  in  contact  with  the  November  meteor  rings. 
When  this  point  is  reached  by  the  earth,  we  have  the 
yearly  display.  If  a  bunch  of  meteors  happens  to  be 
passing  at  that  time,  the  display  is  magnified. 

Meteors  are  also  abundant  on  and  about  the  10th 
of  August.  Unlike  the  November  meteors,  these  are 
about  equally  conspicuous  every  year.  They  are  prob- 


COMETS  AND  METEORS. 


209 


ably  distributed  along  their  ring  uniformly,  so  that 
their  period  of  revolution  has  not  been  certainly  de- 
termined. 


FIG.  52.-ORBIT8  OF  AUGUST  AND  NOVEMBER  METEORS.    (From  Schellen's  Spectrum 
Analysis.) 

199.  Radiant  Point.— When  the  paths  which  a  show- 
er of  meteors  describes  in  the  heavens  are  marked  on 

18* 


210  ASTRONOMY. 


a  celestial  map,  it  is  found  that  if  produced  back- 
ward they  would  all  intersect  nearly  in  one  point.  This 
is  shown  in  Fig.  53.  They  all  seem  to  radiate  from  this 
portion  of  the  heavens,  and  hence  the  name  radiant 
point  is  given  to  it.  The  radiant  point  for  these  meteors 
is  in  the  constellation  Orion.  They  are  therefore  called 
Orionids.  The  radiant  point  of  the  August  meteors  is 
in  Perseus,  and  they  are  hence  called  Perseids.  The 
November  meteors  are  Leonids.  It  must  not  be  inferred 
that  the  meteors  actually  move  in  divergent  lines. 
Their  paths  are  really  parallel ;  but  just  as  railroad- 
tracks  seem  to  approach  each  other  as  they  recede  from 
an  observer,  so  these  parallel  lines  appear  to  radiate 
from  a  common  point.  Just  in  the  radiant  point  the 
meteors  are  seen  without  any  motion,  because  they  are 
directly  approaching  us.  In  the  case  of  the  great  No- 
vember showers,  it  is  stated  that  in  Leo  the  sky  seemed 
phosphorescent  from  the  large  number  of  meteors 
shooting  directly  towards  the  observer. 

200.  How  to  Watch  for  Meteors. — Observations  on 
meteors  are  especially  adapted  to  young  astronomers, 
as  they  require  no  expensive  implements  and  no  skill 
which  cannot  be  easily  acquired  by  a  patient  watcher. 
It  is  important  that  the  position  of  the  radiant  point  be 
accurately  determined,  as  this  distinguishes  the  meteors 
of  one  group  from  those  of  another,  and  is  also  neces- 
sary to  calculate  their  orbit.  The  following  is  the 
method  to  be  employed.  Procure  a  reliable  map  of  the 
heavens,1  and,  spreading  over  it  a  sheet  of  thin,  partly 

1  A  planisphere  set  to  the  middle  of  the  watch  is  convenient  foi 
ordinary  work. 


COMETS  AND  METEORS.  211 

transparent  paper,  mark  on  it  a  number  of  the  prin- 
cipal stars,  being  especially  careful  to  place  those  near 
the  radiant  point  with  great  accuracy.  Mark  also  the 
points  of  the  compass  around  the  horizon.  Now  be- 
come very  familiar  with  the  map  thus  formed,  so  that 
it  will  be  easy  to  find  any  part  of  the  heavens  and  any 
star  quickly.  Whenever  a  meteor  is  noticed,  note  with 
great  accuracy  its  path  in  the  sky,  and  transfer  it  to  the 
map,  indicating  the  length  of  the  track  and  the  direc- 
tion of  the  motion  as  in  Fig.  53.  Notice  also  the 
brightness  of  the  meteor  as  compared  with  Jupiter, 
a  first-magnitude  star,  etc.,  and  write  the  same  by  the 
side  of  the  mark.  Kecord  also  the  exact  time  as 
nearly  as  may  be,  the  color  of  the  meteor,  whether  it 
left  a  streak  behind  it  or  not,  whether  its  course  was 
slow  or  rapid,  and  any  other  interesting  facts  connected 
with  it.  Look  out  especially  for  meteors  with  short 
tracks  near  the  radiant  point,  and  record  them  with 
especial  care ;  and  if  a  meteor  blazes  out  without  any 
track  at  all,  its  position  should  be  exactly  found.  When 
the  watch  is  over,  trace  back  to  their  intersection  the 
various  paths  belonging  to  a  common  system,  and  de- 
termine the  right  ascension1  and  declination1  of  this 
point.  Preserve  the  map  for  future  reference.  Should 
there  be  a  great  shower,  it  is  well  for  one  person  to 
count  the  number  seen  in  each  five  minutes  of  the 
watch,  while  another  records  the  principal  ones.  The 
following  table  shows  the  times  in  the  year  when  the 
earth  passes  through  some  of  the  principal  meteor 
rings,  and  the  radiant  point  for  each: 

1  See  page  42. 


212 


ASTRONOMY. 


Radiant  Point. 

R.A. 

Dec. 

January  2-3. 

232° 

+49° 

Quadrantida. 

April  19-20. 

272° 

+35° 

Lyrids. 

July  27-31. 

337° 

—  6° 

Aquarids. 

August  9-11. 

440 

+66° 

Perseids. 

October  18-20. 

89° 

+15° 

Orionids. 

November  12-14. 

149° 

+23° 

Leonids. 

November  27. 

25° 

+43° 

Andromedes  or  Bielas. 

December  9-12. 

105° 

+31° 

Geminids. 

Instead  of  a  map  a  slated  globe  may  be  used.  This 
is  more  reliable  than  a  map,  but  more  expensive.  It 
may  be  prepared  by  marking  on  it  with  white  paint  the 
circles  of  right  ascension  and  declination,  and  the  posi- 
tions of  the  principal  stars  down  to  the  fourth  magni- 
tude. The  meteor  tracks  may  be  marked  on  this  with 
a  soft  slate-pencil  or  sharp  chalk  and  afterwards  trans- 
ferred to  a  piece  of  paper.  In  all  cases  it  is  well  to 
rule  a  table  something  like  the  following,  and  record 
all  prominent  meteors : 


Beginning 
of  Track. 

End  of 
Track. 

Time. 

Bright- 

Rate  of 

Color. 

Remark*. 

R.  A. 

Dec. 

R.  A. 

Dec. 

November  14, 
1881,  4.20  A  M. 

153° 

+37° 

228° 

+68J0 

1st  mag. 
star. 

Rapid. 

Yellow. 

Leonid;  not 
very  well 

observed  ; 

streak. 

Fig.  53  shows  a  map  of  Orionids  made  at  Haverford 
College  Observatory  on  the  morning  of  October  19, 
1881,  and  is  the  result  of  an  hour's  watch.  The  neigh- 
boring hours  were  quite  as  fruitful  of  meteors  as  this, 
but  the  addition  of  any  more  would  only  confuse.  The 


COMETS  AND  METEORS. 


213 


FIG.  53.— ORIONIDS  OF  OCTOBER  19, 1881. 

general  divergence  of  the  tracks  from  a  point  marked 
by  a  small  circle  is  quite  noticeable.  Some  of  them  do 
not  seem  to  radiate  strictly  from  it,  but  tbis  is  due 
probably  to  errors  of  recording,  a  large  portion  of 
wbicb  are  unavoidable.  Some  of  tbem  belong  to  otber 
sbowers  tban  tbe  Orionids,  and  tbeir  presence  with  the 
others  is  only  accidental.  All  are  given  to  show  the 
appearance  of  such  a  map  at  the  close  of  a  watch. 

201.  Zodiacal  Light. — This  name  is  given  to  a  faint 
light  which  may  be  seen  after  sunset  on  clear  evenings 
of  the  winter  and  spring.  It  is  triangular  in  appear- 


214  ASTRONOMY. 


ance,  its  base  being  on  the  western  horizon,  and, ita, 
greatest  length  extending  back  along  the  path  of  the 
sun;  it  may,  on  favorable  evenings,  be  observed  to 
extend  nearly  to  the  meridian,  where  it  fades  away  so 
that  no  distinct  outline  may  be  noticed.  Some  people 
with  very  good  eyes  claim  to  be  able  to  see  it  all  the 
way  to  the  eastern  horizon.  It  really  exists  in  the 
summer  and  autumn  as  well,  but  in  our  latitude  the 
ecliptic1  lies  so  near  to  the  horizon  at  these  times  that 
the  light  cannot  be  easily  distinguished.  It  is  then£ 
however,  visible  in  the  morning  just  before  sunrise  on 
the  other  side  of  the  sun.  It  is  not  certainly  known 
what  it  is,  but  the  most  probable  theory  is  that  it  is 
composed  of  an  immense  number  of  meteoroids,  re- 
flecting the  sunlight,  and  which  are  so  small  that  their 
united  lustre  is  barely  distinguishable.  They  surround 
the  sun  on  all  sides,  revolving  about  him  like  little 
planets,  and  frequently  fall  to  his  surface,  thus  assisting 
in  keeping  up  his  heat  and  light,  as  explained  on  page 
62.  The  light  should  be  looked  for  from  a  half-hour 
to  an  hour  before  sunrise  or  after  sunset. 

Relation  between  Comets  and  Meteors. 

202.  Prof.  Newton,  of  Yale  College,  and  Prof.  Adams, 
of  England,  entered  into  an  elaborate  investigation  to 
find  the  orbit  of  the  November  meteors.  They  based 
their  work  on  the  old  records  of  former  displays  and 
the  observations  of  the  shower  of  1866.  The  result  of 
their  labors  was  to  lay  down  very  accurately  the  size 
and  position  of  the  orbit.  When  this  was  done  it  was 

1  See  page  39. 


COMETS  AND  METEORS.  215 

found  that  it  agreed  almost  exactly  with  that  of  a  small 
comet  commonly  known  as  Comet  I.,  1866,  or  Terapel's 
comet,  which  also  had  a  period  of  about  thirty-three 
and  one-fourth  years,  and  which  returned  to  perihelion 
in  the  early  part  of  the  same  year  in  which  the  brilliant 
meteoric  display  occurred.  It  was  thus  found  that  the 
main  group  of  November  meteors  followed  around  in 
the  orbit  of  the  comet ;  that  the  earth  met  the  comet 
about  ten  months  before  the  meteoric  swarm ;  and  that 
the  comet  led  the  way  round  and  round  the  sun,  with 
the  swarm  immediately  and  continually  following  it. 
Fig.  55  shows  the  identity  of  the  two  orbits. 

But  this  is  not  an  isolated  instance.  It  was  soon 
found  that  the  August  meteors  and  Comet  IH.,  1862, 
had  identical  orbits,  and,  later,  a  similar  relation  was 
found  to  exist  between  the  Lyrids  of  April  20  and 
Comet  L,  1861.  Another  interesting  case  is  that  of 
Biela's  comet.  We  have  said  (page  201)  that  in  1866 
it  was  expected,  but  that  it  never  appeared.  The  next 
return  would  have  been  in  1872.  On  the  night  of 
November  27  the  earth  and  the  comet,  it  was  calcu- 
lated, would  be  at  the  same  point  at  nearly  the  same 
time.  But  there  came,  instead  of  the  comet,  a  shower 
of  meteors.  They  rained  down  on  England  at  the  rate 
of  over  ten  thousand  an  hour ;  they  brightened  up  the 
earth  and  sky,  and  many  an  observer  recorded  the  fact 
that  they  all  radiated  from  the  same  point  in  the  con- 
stellation Andromeda,  and  that  point  was  just  where 
the  comet  was  expected  to  come  from.  The  comet  had 
gone  no  one  knows  where,  but  a  swarm  of  meteors  had 
assumed  its  place.  Every  year,  on  the  27th  of  No- 
vember, the  shower  may  be  seen,  and  its  brightness 


216  ASTRONOMY. 


increases  as  the  time  for  the  regular  return  of  Biela's 
comet  approaches. 

These  coincidences  cannot  be  attributed  to  chance. 
There  must  be  some  connection  between  comets  and 
meteors,  but  what  it  is  astronomers  have  not  certainly 
determined.  "We  should  keep  on  observing  facts,  trust- 
ing that  soon  the  mystery  will  be  thoroughly  cleared 
up.  It  is  probable  that  the  solid  portion  of  the  comet 
has  been  broken  up  by  some  internal  convulsion,  and 
that  the  meteors  are  the  fragments. 


PART  II. 

THE   SIDEREAL   UNIVERSE. 


CHAPTER   I. 

THE   CONSTELLATIONS. 

203.  Introductory. — So  far  we  have  kept  within  the 
limits  of  the  solar  system.  We  have  been  struck  with 
the  immense  intervals  of  space  which  separate  its  dif- 
ferent members.  The  distance  to  the  moon  is  greater 
than  anything  we  can  conceive  of,  yet  it  is  but  a  trifle 
when  compared  with  the  distance  to  the  sun ;  but  even 
the  earth's  orbit  seems  small  when  we  think  of  the  enor- 
mous length  of  the  path  which  Neptune  passes  over  in 
each  revolution  about  the  sun.  We  now  are  about  to 
consider  bodies  whose  distance  is  so  great  that  the 
huge  orbit  of  Neptune  seems  but  a  point  in  comparison. 
When  we  gain  some  familiarity  with  them,  the  solar 
system,  great  as  it  is,  will  seem  to  us  like  a  little  com- 
pany of  orbs,  near  at  home,  clustered  together  in  infi- 
nite space;  a  very  insignificant  portion  of  the  whole 
universe.  These  distant  bodies  are  the  stars  and  the 
nebulae.  Taken  as  a  whole,  they  constitute  the  sidereal 
system.  This  system  embraces  all  the  heavenly  bodies. 
The  solar  system  is  a  part  of  it,  to  us  a  very  conspicu- 

19  217 


218  ASTRONOMY. 


ous  part,  but,  compared  with  the  whole,  quite  diminu- 
tive. We  must  not  include  under  the  name  stars  the 
various  members  of  our  solar  system. 

Though  Jupiter  and  Venus  and  the  other  planets 
resemble  stars  to  the  eye,  they  really  differ  widely 
The  planets  revolve  around  the  sun  like  the  earth ;  the 
stars  do  not.  The  planets  are  comparatively  close  to 
us ;  the  nearest  star  is  more  than  seven  thousand  times 
as  far  from  the  sun  as  is  Neptune.  The  planets  shine 
by  the  light  which  they  reflect  to  us  from  the  sun ;  the 
stars  give  out  their  own  light. 

204.  What  are  Stars  ? — The  stars  are  suns.  They  give 
out  light  and  heat  like  the  sun.  As  soon  as  we  come 
to  treat  of  the  distance  to  the  stars  we  shall  see  that  it 
would  be  impossible  to  consider  that  they  receive  the 
light  which  they  send  us  from  the  sun  as  the  planets 
do.  They  must  be  hot  and  glowing  bodies  themselves, 
some  of  them  as  large  and  as  bright  as  the  sun,  and 
some  of  them  probably  much  larger  and  brighter.  If 
we  were  to  look  at  the  sun  from  their  distance,  it  would 
appear  to  be  a  little  point  of  light  as  they  do.  The  sun 
is  a  star.  "We  must  consider  space  to  be  occupied  with 
a  countless  number  of  suns  scattered  very  thinly  through 
it ;  and,  though  we  have  never  seen  any  worlds  sur- 
rounding any  sun  but  our  own,  it  is  very  probable  that 
they  exist,  and  that  each  star  is  the  centre  of  a  system 
to  some  extent  resembling  the  solar  system.  There  is 
another  proof  of  the  fact  that  the  stars  are  like  the  sun. 
The  spectroscope  tells  the  same  story  of  both :  they 
both  consist  of  a  glowing  mass,  the  light  from  which 
shines  through  a  gaseous,  less  bright  atmosphere,  and 
the  materials  of  which  this  atmosphere  consist  are 
largely  the  same  in  all. 


THE  CONSTELLATIONS.  211) 

The  ancients  had  in  general  very  incorrect  ideas  as 
to  what  the  stars  were.  They  were  variously  supposed 
to  be  studs  nailed  to  the  celestial  sphere;  fires  which 
were  nourished  by  the  igneous  matter  which  streamed 
out  from  the  centre  of  the  earth;  luminous  stones 
whirled  up  from  the  earth ;  breathing-holes  in  the  uni- 
verse. Pythagoras  had  a  more  exalted  idea  of  them,  con- 
sidering them  to  be  worlds  having  land,  water,  and  air. 

205.  Constellations. — In    very   remote    antiquity  the 
heavens  were  divided  up  into  groups  of  stars  which 
were  called  constellations.     Names  were  given  to  some 
of  these,  probably  on  account  of  their  fancied  resem- 
blance to  certain  animals  and  other  objects,  though  it 
is  difficult  to  see  any  such  resemblance  now.      The 
seven  stars  commonly  called  the  Dipper  are  a  part  of 
the  constellation  "  Great  Bear"  yet  the  arrangement  of 
the  stars  in  this  constellation  does  not  seem  to  suggest 
anything  of  the  kind.     Other  names,  as  Hercules,  were 
probably  given  for  the  sake  of  honoring  their  deities 
or  great  men.     Astronomers  have  found  it  convenient 
to  retain  these  names  to  mark  certain  portions  of  the 
heavens.     Thus,  Leo  does  not  now  mean  a  great  lion 
or  anything  resembling  one,  but  a  definite  section  of 
the  celestial  sphere  containing  certain  stars. 

206.  Names  of  the  Stars. — The  heavens  being  thus 
divided  up  into  small  areas,  with  a  name  for  each,  it 
remained  to  adopt  some  plan  for  distinguishing  one 
star  from  others  in  the  same  constellation.    The  method 
now  in  use  was  suggested  by  Bayer^  in  1603.     The 
brightest  star  of  the  constellation  is  named  by  prefixing 


1  Bayer,  a  German  astronomer  and  Protestant  preacher,  1572  to 
1660. 


220  ASTRONOMY. 


the  first  letter  of  the  Greek  alphabet  to  the  genitive  case 
of  the  name  of  the  constellation.  Thus,  the  brightest 
star  in  the  Great  Bear  is  a  Ursce  Majoris.  The  brightest 
in  Leo  is  a  Leonis.  The  other  Greek  letters  and  the  Ro- 
man letters  then  follow  somewhat  indiscriminately ;  and 
if  this  does  not  exhaust  the  stars  of  any  constellation,  the 
remaining  ones  are  numbered.  This  order  is  not  strictly 
correct;  the  namers  sometimes  misjudged  the  brightness 
of  the  stars,  and  it  is  probable  that  the  splendor  of  some 
of  them  has  changed  since  the  names  were  given.  Thus, 
0  Orionis  is  in  general  brighter  than  «  Orionis,  but 
sometimes  the  latter  greatly  exceeds  it  in  light.  Be- 
sides this,  the  stars  in  the  different  constellations  have 
been  independently  numbered,  so  that  a  single  star  is 
sometimes  designated  by  its  letter  as  well  as  by  its  num- 
ber. This  double  system  often  causes  confusion.  Many 
hundreds  of  the  stars  had  received  special  names  from 
the  ancients,  particularly  from  the  Arabians ;  but  the 
inconvenience  of  remembering  these  has  kept  them 
from  being  extensively  used,  except  in  the  case  of  a 
few  of  the  most  conspicuous  of  them.  Thus,  «  Bootis 
is  Arcturus,  «  Lyrse  is  Vega,  etc. 

207.  Magnitudes. — For  convenience  of  classification 
the  stars  have  been  divided  according  to  their  bright- 
ness into  magnitudes,  the  first-magnitude  stars  being 
the  brightest.  Stars  of  the  first  five  magnitudes  can  be 
seen  with  the  naked  eye,  and  on  favorable  nights  many 
stars  of  the  sixth  magnitude  may  also  be  seen.  The 
number  in  each  of  the  first  six  magnitudes  is  approxi- 
mately given  in  the  following  table : 

1st    ...      20  4th ...      450 

2d     .        .        .      65  5th.        .        .     1100 

3d  200  6th.  ,    4000 


THE  CONSTELLATIONS.  221 

It  thus  appears  that  there  are  about  6000  stars  which 
under  the  most  favorable  conditions  can  be  seen  by  the 
naked  eye.  Some  of  these  are  never  above  the  horizon 
in  middle  latitudes,  and  only  a  part  of  the  remainder 
are  above  at  any  given  time.  Many  eyes  cannot  see 
the  fainter  ones  of  the  sixth  magnitude,  so  that  2000 
will  cover  all  commonly  seen,  by  most  people,  at  any 
one  time.  Telescopes  reveal  them  by  the  millions.  Fig. 
54  shows  the  appearance  of  a  portion  of  the  heavens  as 
seen  by  a  telescope.  There  are  not  more  than  two 
stars  here  visible  to  the  naked  eye. 

Sir  William  Herschel  gives  the  following  table  as 
representing  the  light  given  out  lay  a  star  of  the  differ- 
ent magnitudes,  an  average  sixth-magnitude  star  being 
taken  as  unity : 


6th      .       .       .1 

5th       .        .        .2 
4th  ,     6 


3d  .  .  .12 
2d  .  .  .25 
1st  ,  100 


As  a  general  rule,  an  average  star  of  any  magnitude 
is  about  two  and  one-half  times  as  bright  as  an  average 
star  of  the  magnitude  next  fainter. 

It  must  not  be  supposed  that  all  stars  of  the  same 
magnitude  are  equally  brilliant.  There  are  all  grades 
of  brightness,  from  Sirius,  the  light  from  which  is  esti- 
mated to  be  234  times  as  great  as  that  from  a  sixth- 
magnitude  star,  to  those  so  faint  as  scarcely  to  be  vis- 
ible. There  is  no  distinct  line  to  be  drawn  between 
the  different  magnitudes.  A  star  lying  between  the 
fourth  and  fifth  magnitudes,  for  example,  might  be 
considered  to  be  a  faint  star  of  the  fourth  by  some 
astronomers  and  a  bright  star  of  the  fifth  by  others. 
These  intermediate  stars  are  often  designated  by  deci- 


222  ASTRONOMY. 


mals.     Thus,  magnitude  4.8  would  mean  nearer  the 
fifth  than  the  fourth. 

208.  The  following  list  contains  the  names  of  twenty 
of  the  most  brilliant  stars  of  the  heavens  arranged 
nearly  in  the  order  of  brightness ;  those  in  italics  are 
never  seen  in  the  latitude  of  New  York : 


a  Canis  Majoris,  or  Sir'ius. 

a  Argus,  or  Cano'pus. 

a  Centauri. 

a  Bootis,  or  Arctu/rus. 

/?  Orionis,  or  Ri'gel. 

a  Aurigae,  or  Capel'la. 

a  Lyrse,  or  Ye'ga. 

a  Canis  Minoris,  or  Pro'cyon. 

a  Orionis,  or  Betel'geuse. 

a  Eridani,  or  Acher'nar. 


a  Tauri,  or  Aldeb'aran. 
0  Centauri. 
a  Crucis. 

I  a  Scorpii,  or  Antar'es. 
a  Aquilae,  or  Altair'. 
a  Yirginis,  or  Spi'ca. 
a  Piscis  Australis,or  Fo/malhaut. 
fi  Crucis. 

(3  Geminorum,  or  Pol'lux. 
a  Leonis,  or  Reg'ulus. 


These  should  all  be  found  on  a  celestial  globe  or 
map,  and  then  in  the  heavens.  They  will  serve  as  val- 
uable starting-points  from  which  to  locate  the  fainter 
stars.  To  give  examples  of  stars  of  the  lower  magni- 
tudes, we  will  go  over  the  stars  of  the  Dipper  : 

The  brightest  of  the  pointers  is  of  the  second  mag- 
nitude ;  the  other  pointer  is  of  the  third ;  the  star  next 
to  this  is  also  of  the  third;  the  star  which  joins  the 
handle  of  the  dipper  to  the  bowl  is  of  the  fourth ;  the 
next  star  in  the  handle  is  of  the  third ;  the  next,  of  the 
second ;  and  very  close  to  it  is  one  of  the  fifth ;  the 
last  one  in  the  handle  is  of  the  third.  On  a  moonless 
night,  with  a  clear  atmosphere,  there  are  three  little 
stars  of  the  sixth  magnitude  which  may  be  seen  near 
the  star  at  the  end  of  the  handle  of  the  dipper,  and  in 
the  direction  of  the  pole-star. 

Star  Maps  and  Stellar  Photography. — It  is  necessary 


THE  CONSTELLATIONS.  223 

for  any  one  who  has  work  to  do  with  the  stars  to  have 
accurate  maps  of  the  heavens.  Several  of  these,  em- 
bracing all  visible  stars  and  some  others,  have  been 
made,  and  are  very  good.  The  best  way  to  make  these 


FIG.  54. — PART  OF  THE  CONSTELLATION  CTOKUS,  AS  SEEN  WITH  A  TELISOOPK. 

maps  is  by  photography.  The  telescope  is  pointed  to 
a  portion  of  the  heavens,  and  the  image  is  allowed  to 
fall  on  a  photographic  plate  instead  of  the  eye.  Every 
star  leaves  its  impression  in  just  the  right  relative  po- 


224  ASTRONOMY. 


sition  ;  and  it  is  a  curious  fact  that  stars  too  faint  to  be 
seen  by  the  eye  will  impress  their  images  on  the  plate 
if  the  exposure  is  continued  long  enough. 

A  company  of  astronomers,  scattered  over  the  world 
at  eighteen  different  observatories,  is  now  at  work  to 
secure  maps  of  all  parts  of  the  heavens.  Several  years 
will  be  required  to  perform  the  task.  When  com- 
pleted the  most  accurate  and  exhaustive  star-charts 
ever  made  will  be  at  the  service  of  astronomers.  About 
twenty  thousand  plates  will  be  made  of  the  heavens. 
Another  such  series,  taken  a  few  years  later,  will  show 
very  satisfactorily  any  changes  in  the  relative  position 
of  the  stars. 

209.  The  Milky  Way,  or  Galaxy.— This  is  a  ring  of 
hazy  light,  which  seems  to  encircle  the  earth,  and  is 
visible  on  moonless  nights.  By  the  telescope  it  is 
shown  to  consist  of  millions  of  stars  clustered  together. 
They  are  either  so  small  or  so  distant  that  we  cannot 
see  them  separately  by  the  unassisted  eye.  The  Milky 
Way  partakes  of  the  diurnal  motion  of  the  heavens,  and 
is  therefore  seen  in  varying  positions  at  different  times. 
But  it  never  changes  its  place  among  the  stars.  The 
telescope  shows  that  nearly  all  the  faint  stars  are  clus- 
tered in  or  around  it.  We  see  but  few  near  its  poles,1 
but  as  we  approach  they  increase  in  number  very  rap- 
idly. A  careful  observer  can  see  this — less  conspicu- 
ously, though — with  regard  to  the  brighter  stars.  A 
large  majority  of  all  the  stars  are  clustered  in  or  near 
the  plane  of  the  Milky  Way. 

In  the  Galaxy  itself  the  stars  are  not  distributed  uni- 
formly, but  grouped  in  large  or  small  clusters,  with 

1  Points  90°  from  it. 


THE  CONSTELLATIONS.  225 

blank  spaces  between  them.  The  edge  is  jagged  and 
irregular,  and  wherever  there  is  an  outlying  streamer 
there  the  lucid  stars  seem  also  to  be  congregated,  so  as 
to  suggest  some  connection.  At  one  point  it  divides 
into  two  parts,  which  afterwards  join  each  other. 

210.  Distances  of  the  Stars. — If  a  person  were  walking 
past  a  grove  of  trees,  they  would  seem  to  him  to  be  con- 
tinually changing  their  places  among  one  another ;  the 
trees  nearest  him  would  appear  to  move  backward,  with 
respect  to  those  beyond ;  two  trees  which  were  in  a  line 
with  him  at  one  instant  would  seem  to  separate  as  he 
advanced ;  others  would  appear  to  approach  one  another. 
Now,  the  earth  is  sweeping  through  space  around  the 
sun,  passing  every  six  months  from  one  side  of  its  orbit 
to  the  opposite.  These  points  are  separated  from  each 
other  about  186,000,000  miles.  It  would  seem,  then, 
that  the  stars  ought  to  change  their  apparent  places 
among  one  another  just  as  the  trees  do ;  in  other  words, 
that  the  stars  ought  to  show  some  parallax.  Evidences 
of  this  were  sought  with  great  care  for  a  long  time 
without  any  success.  The  stars  were  so  distant  that 
any  change  of  position  was  inappreciable.  With  in- 
struments capable  of  greater  precision  a  very  small 
parallax  has  in  modern  times  been  detected  for  certain 
stars,  though  in  no  case  has  it  been  found  to  equal  one 
second  of  arc.  Let  us  consider  the  meaning  of  this. 
Suppose  that  an  observer  were  on  the  nearest  star,  with 
a  telescope  steadily  pointing  to  the  earth  as  it  moved 
from  one  side  to  the  other  of  its  immense  orbit.  The 
direction  of  the  telescope  at  the  beginning  of  the  watch 
would  deviate  from  that  at  the  end  by  an  angle  of  one 
and  a  half  seconds.  The  radius  of  the  earth's  orbit 
would  subtend  an  angle  of  only  three-quarters  of  one 


226  ASTRONOMY. 


second ;  if  it  were  a  luminous  rod,  it  would  appear  no 
longer  than  a  foot-rule  would  at  the  distance  of  about 
fifty  miles.  A  calculation  shows  that  the  distance  to 
this  star  is  about  25,000,000,000,000  miles ;  but  there 
is  no  advantage  in  expressing  a  distance  as  great  as 
that  to  the  stars  in  miles.  The  number  is  utterly  in- 
conceivable. If  we  take  the  distance  to  the  sun  as  our 
unit,  or  measuring- rod,  there  would  be  264,000  such 
units  in  the  line  joining  us  to  the  nearest  star.  Such 
distances  are  usually  measured  by  the  time  it  takes 
light  to  pass  over  them.  Light  moves  with  a  velocity 
of  about  186,000  miles  per  second.  It  would  take  it 
about  one  and  a  third  seconds  to  pass  between  the  earth 
and  the  moon ;  in  a  little  over  eight  minutes  it  comes  to 
us  from  the  sun ;  but  to  reach  us  from  the  nearest  fixed 
star  over  four  years  are  required,  while  its  time  in  com- 
ing from  many  of  the,  stars  is  measured  by  centuries. 

The  nearest  known  star,  a  Centauri,  in  the  Southern 
hemisphere,  is  a  very  bright  one.  The  next  one,  61 
Cygni,  is  only  of  the  sixth  magnitude.  It  is  not  found, 
as  a  rule,  that  the  brightest  stars  are  the  nearest. 
Sirius,  the  most  brilliant  star  of  the  whole  heavens,  is 
three  times  as  far  away  as  61  Cygni.  This  requires  us 
to  suppose  that  the  stars  are  of  different  sizes.  Sirius 
must  be  very  much  larger  than  61  Cygni  to  give  out  so 
much  more  light  at  a  greater  distance.  Knowing  the 
relative  distances  from  us  of  Sirius  and  the  sun,  and 
the  light  given  by  each,  we  can  calculate  that,  sup- 
posing their  surfaces  to  be  equally  bright,  Sirius  must 
have  about  fifty-six  times  as  much  surface  as  the  sun, 
and  hence  over  seven  times  as  long  a  diameter. 

Though  the  stars  are  such  large  bodies,  they  show  no 
diameters  in  the  most  powerful  telescopes.  They  are 


THE  CONSTELLATIONS.  227 

merely  points  of  light,  brighter,  but  no  larger,  than 
when  seen  by  the  eye.  This  is  shown  by  an  occulta- 
tion  of  a  star  by  the  moon.  (Page  139.) 

211.  Motions  of  the  Stars. — The  ancients  called  the 
stars  fixed,  because  they  did  not  seern  to  change  their 
places  among  one  another  as  the  planets  did.  By  com- 
paring their  relative  positions  now  with  what  they  were 
when  the  first  star  catalogues  were  made,  it  is  found  that 
many  of  them  have  very  considerable  motion.  This 
motion  of  the  stars  among  one  another  is  called  proper 
motion.  In  the  course  of  an  immensely  long  period  the 
constellations  will  be  distorted.  As  an  illustration  of 
how  this  is  possible,  we  may  take  the  case  of  the  Great 
Bear.  Of  the  seven  principal  stars  forming  the  Dipper, 
it  has  been  found  that  five  are  moving  in  parallel  lines 
and  with  equal  velocities.  Their  relative  positions 
will  therefore  be  preserved ;  but  the  two  pointers  are 
moving  in  opposite  directions.  After  centuries  have 
elapsed  they  will  cease  to  point  to  the  pole-star,  and 
the  Dipper  will  change  its  shape. 

This  motion  can  be  detected  only  by  very  delicate 
measurements.  It  is  probably  extremely  rapid,  but 
the  great  distance  makes  it  appear  to  be  very  minute. 
We  must  also  remember  that  the  stars  are  not  bodies 
at  the  same  distance  from  us,  set  in  the  surface  of  the 
same  sphere,  and  moving  in  that  surface,  but  are  at 
varying  distances,  and  moving  in  all  directions,  towards 
us  and  away  from  us,  as  well  as  sidewise.  The  latter  we 
might  hope  to  detect,  but,  until  recently,  motion  to  us 
or  away  from  us  appeared  hopelessly  undiscoverable. 
But  the  spectroscope  has  enabled  us  to  solve  the  diffi- 
culty, and  we  can  now  tell  with  reasonable  accuracy 
which  way  nearly  all  the  bright  stars  are  moving. 


228  ASTRONOMY. 


Like  the  other  stars,  the  sun  has  a  proper  motion, 
carrying  with  it  the  whole  solar  system.  This  has  been 
proved  by  the  apparent  motion  of  the  stars.  Let  us 
recur  to  our  illustration  of  the  grove.  Suppose  the 
observer  to  be  directly  approaching  it :  the  trees  would 
then  appear  to  be  separating  from  one  another.  If  he 
were  moving  away  from  it,  the  trees  would  seem  to 
close  up.  Now,  if  in  any  part  of  the  heavens  the  stars 
are  apparently  opening  out,  and  in  the  opposite  portion 
closing  up,  it  is  evidence  that  we,  in  common  with  the 
rest  of  the  solar  system  and  the  sun,  are  approaching 
the  centre  of  the  former  part.  This  point  has  been 
calculated  by  various  astronomers,  and  they  agree  in 
placing  it  in  the  constellation  Hercules. 

212.  Colors  of  the  Stars. — It  is  easily  observed  that 
the  stars  vary  in  color.  Any  one  carefully  noticing 
the  colors  of  Sirius  and  Betelgeuse,  for  example,  would 
not  fail  to  see  a  striking  difference.  They  are  usually 
either  blue,  white,  yellow,  or  red.  Of  the  stars  which 
have  a  blue  or  green  tinge,  Sirius,  Vega,  and  Rigel 
may  be  mentioned;  of  the  white  stars,  Regulus  and 
Polaris;  of  the  yellow,  Capella  and  the  sun;  of  tha 
red,  Antares  and  Betelgeuse.1  There  are  also  a  num- 
ber of  faint  telescopic  stars  which  are  of  a  deep  blood- 
red  color ;  these  are  usually  remarkable  for  changes  in 
their  brightness.  The  student  should  carefully  exam- 

1  J.  Norman  Lockyer,  a  noted  living  English  astronomer,  has  pro- 
pounded the  theory  that  the  colors  of  the  stars  indicate  the  intensity 
of  their  heat.  Just  as  a  piece  of  metal  goes  through  its  changes  from 
red-hot  to  white-hot  as  the  temperature  is  raised,  so  the  red  stars  are 
the  least  heated  of  all  visible  stars.  Then  follow  in  order  the  yellow 
the  white,  and  the  hlue.  This  is  not  fully  established. 


THE  CONSTELLATIONS.  229. 

ine  these  and  other  stars  and  make   out  lists  of  the 
colors  of  all  the  conspicuous  ones. 

The  telescope  shows  sometimes  a  beautiful  collection 
of  stars  of  different  colors  grouped  together.  Such  a 
cluster  in  the  Southern  hemisphere,  Sir  John  Herschel 
said,  produced  on  his  mind  the  effect  of  a  superb  piece 
of  fancy  jewelry.  Frequently  a  red  and  a  blue  star  are 
associated  together  in  close  contrast. 

213.  Twinkling  of  the  Stars. — One  of  the  most  notice- 
able features  about  the  stars  is  their  twinkling.  This 
is  due  to  the  different  temperatures  and  densities  of  the 
strata  of  the  atmosphere  through  which  their  light 
passes.  We  know  this  from  the  fact  that  stars  near  the 
horizon,  the  light  from  which  traverses  a  greater  stretch 
of  atmosphere,  twinkle  more  than  those  in  the  zenith. 
But  there  is  some  other  cause  connected  with  it,  for 
some  stars  at  the  same  altitude  twinkle  more  than 
others.  If  Castor  and  Pollux  be  watched  when  they 
are  near  the  horizon,  it  will  be  noticed  that  Pollux 
twinkles  the  most.  The  cause  of  this  difference  is  not 
known. 

On  the  nights  when  the  stars  twinkle  the  most  the 
greatest  number  of  faint  stars  are  visible,  though  for 
telescopic  work  such  nights  are  apt  to  be  poor,  on  ac- 
count of  the  unsteadiness  of  the  atmosphere. 

20 


230  ASTRONOMY. 


Description  of  the  Constellations.1 

214.  The  Star  Maps,  given  on  pages  237-248,  will 
enable  the  student  to  trace  out  the  position  of  the  con- 
stellations and  brighter  stars.     As  the  heavens  are  con- 
tinually changing  their  position  relative  to  the  horizon 
as  a  result  of  the  diurnal  motion,  it  may  be  difficult  at 
first  to  ascertain  what  portion  is  represented  by  any 
map.     The  Pole  Star  and  the  Dipper  may  be  found, 
and  the  rest  grouped  around  these.     Each  map  con- 
tains near  its  edges  the  stars  of  the  adjoining  map,  so 
they  can  be  to  some  extent  fitted  together.     Maps  1,  2, 
and  3  represent  the  part  of  the  heavens  around  the 
North  Pole  in  different  positions.     Maps  4,  5,  6,  and  7 
represent  a  strip  of  the  sky  lying  just  north  of  the 
celestial  equator,  while  the  remaining  five  maps  show 
a  similar  strip  south  of  the  equator. 

215.  The  circles  of  declination  and  the  meridians  are 
given  on  the  maps,  so  that  any  object  whose  right  as- 
cension and  declination  are   known   can   be  located. 
Also,  if  the  position  of  any  body  is  desired,  it  can  be 
found  on  the  map  and  its  right  ascension  and  declina- 
tion read  off  by  means  of  the  little  figures  on  the  edges. 

216.  We  will  start  with  the  Dipper.     Take  one  of 
the  first  three  maps,  find  the  Dipper  in  Ursa  Major, 
and  hold  it  up  so  as  to  agree  with  its  position  in  the 
sky.     The  following   description   will   enable   you  to 
trace  the  other  constellations  around  the  pole,  using 

1  There  is  no  advantage  in  studying  this  section  except  in  connec- 
tion with  the  maps  and  the  heavens.  Remembering  the  location  of 
the  stars  without  finding  them  in  the  sky  will  be  utterly  unprofitable. 


THE  CONSTELLATIONS.  231 

whichever  one  of  the  three  covers  the  part  of  the  sky 
you  are  searching  in.  All  of  the  sky  within  40°  of  the 
pole  will  be  visible  above  the  horizon  in  the  latitude 
of  Philadelphia,  and  still  more  farmer  north. 

Ursa  Minor  embraces  Polaris.  In  it  may  be  found 
the  "  Little  Dipper,"  with  the  pole-star  in  the  end  of 
the  handle. 

Cassiopeia  is  directly  opposite  Polaris  from  the  Great 
Dipper.  The  conspicuous  part  is  an  irregular  W,  in 
which  its  five  brighter  stars  are  arranged.  The  ar- 
rangement also  bears  some  resemblance  to  a  chair,  and 
the  ancients  represented  Cassiopeia  as  a  queen  seated 
on  a  throne.  If  a  line  be  drawn  from  the  faintest  star 
of  the  seven  in  the  Dipper  to  B  in  Cassiopeia,  Polaris 
will  be  nearly  in  the  middle  of  this  line. 

The  two  stars  ?  and  8  Cassiopeia  point  to  a  cluster 
which  to  the  naked  eye  seems  a  mere  haze  of  light, 
but  which  a  small  telescope  resolves  into  a  beautiful 
collection  of  stars.  This  is  in  the  constellation  Per- 
seus. Continuing  this  line  as  much  farther,  we  come 
to  several  stars  of  medium  brightness,  which  consti- 
tute the  principal  part  of  the  constellation.  0  Persei, 
or  Algol,  is  a  little  farther  from  the  pole  than  this 
group.  Its  peculiarity  will  be  explained  in  the  next 
chapter.1 

The  constellation  Auriga  adjoins  Perseus  on  the  side 


1  According  to  ancient  mythology,  Cassiopeia  was  the  queen  of 
Cepheus,  King  of  Ethiopia.  Becoming  vain  of  her  beauty,  she  boasted 
that  she  was  fairer  than  Juno,  the  sister  of  Jupiter,  or  than  the  sea- 
nymphs.  To  punish  her  presumption,  it  was  ordained  that  she  should 
chain  her  daughter,  Andromeda,  whom  she  tenderly  loved,  to  a  desert 
rock,  exposed  to  the  fury  of  a  sea-monster.  But  Andromeda  was 
rescued  and  the  monster  destroyed  by  the  great  warrior  Perseus. 


232  ASTRONOMY. 


away  from  Cassiopeia.  It  has  two  stars  of  considerable 
brightness,  the  brightest  being  the  first-magnitude  star 
Capella. 

Only  one-half  of  the  other  maps  are  available  at  one 
time.  The  remainder  represent  stars  below  the  hori- 
zon. The  student,  after  working  out  the  circumpolar 
stars,  will  have  little  difficulty  in  finding  the  map  he 
wants.  The  descriptions  which  follow  will  apply  to 
the  heavens  as  they  appear  at  8.30  o'clock  on  January 
1st,  April  1st,  July  1st,  and  October  1st. 

217.  Southern  Constellations  Visible  in  Winter. — Cas- 
siopeia is  now  west  of  the  zenith,  and  the  Great  Dipper 
is  rising  in  the  east, 

The  Milky  Way  extends  across  the  heavens  from  the 
northwest  to  the  southeast,  passing  through  the  zenith. 
Just  on  its  edge,  in  the  east,  is  the  most  brilliant  con- 
stellation of  the  northern  heavens,  Orion,  which  on  old 
star  maps  is  represented  by  the  figure  of  a  hunter.  Four 
bright  stars  form  an  irregular  four-sided  figure.  One 
of  these,  a  red  star  of  the  first  magnitude,  in  the  shoul- 
der, is  Betelgeuse  (*  Oriouis).  The  other  first-magni- 
tude star  is  Rigel  (P  Orionis),  in  the  foot.  Three  stars 
ot  the  third  magnitude  lie  in  a  row  in  the  belt.  Below 
the  belt  is  another  row  of  three  fainter  stars,  the  cen- 
tral one  of  which  is  surrounded  by  the  famous  nebula 
of  Orion.  On  a  moonless  night  this  may  be  seen  as  a 
faint  haze  by  the  naked  eye. 

Taurus  lies  between  Orion  and  the  zenith.  The 
group  Hyades  is  easily  known  by  its  Y-shape.  The  first- 
magnitude  star  at  one  extremity  of  the  V  is  Aldebaran. 
Farther  away  from  Orion  are  the  Pleiades,  a  little  clus- 
ter which  cannot  be  mistaken. 

Canis  Major  lies  opposite  Orion  from  Taurus.     The 


THE  CONSTELLATIONS.  233 

very  bright  Sirius  is  its  brilliant  ornament.  Two 
second-magnitude  stars  are  about  11°  southeast  of 
Sirius. 

Procyon,  in  Canis  Minor,  makes  a  nearly  equilateral 
triangle  with  Sirius  and  Betelgeuse.1 

About  half-way  between  Orion  and  Polaris  is  Ca- 
pella,  the  brightest  star  of  the  constellation  Auriga.  A 
little  farther  from  the  zenith  is  p  Aurigse. 

Gemini  lies  between  Capella  and  the  east.  Castor 
and  Pollux,  two  bright  stars  about  4°  apart,  are  easily 
distinguished.  Between  Castor  and  Polaris  is  a  barren 
part  of  the  heavens. 

The  variable  star  Algol  will  be  almost  exactly  in  the 
zenith,  with  the  rest  of  Perseus  a  little  to  the  north. 

Eridwnus  is  a  large  constellation  south  of  Taurus, 
which  does  not  contain  any  very  conspicuous  stars. 

CetuSy  west  of  Eridanus,  is  also  a  large  constellation. 
It  contains  two  stars  of  the  second  magnitude,  and  the 
variable  Mira. 

On  the  western  side  of  the  zenith  the  most  conspicu- 
ous group  is  the  "  Square  of  Pegasus."  This  consists 
of  four  stars  in  the  form  of  a  large  quadrilateral,  so 
arranged  that  two  pairs  point  nearly  to  Polaris.  The 
brightest  of  these,  that  one  nearest  the  zenith,  is  not 
in  Pegasus,  but  is  «  Andromedse.  The  others  are  a,  ft, 
and  f  Pegasi.  Between  «  Andromedae  and  Algol  is  /3 
Andromedse. 

The  above  description  will  also  answer  for  10.30 
o'clock,  December  1 ;  12.30,  November  1;  and  so  on. 


1  Taurus  is  usually  represented  as  a  bull  charging  at  Orion,  while 
the  Hunter's  two  dogs,  Canis  Major  and  Canis  Minor,  follow  him 
around  the  sky. 

20* 


234  ASTEONOMY. 


218.  Southern    Constellations   Visible  in  Spring. — The 
Pointers  will  now  be  nearly  overhead,  and  Cassiopeia 
low  down  in  the  northwest.     Orion  and  Taurus  will  be 
sinking  to  the  western  horizon.     Sirius  will  be  low  in 
the  southwest,  and  Procyon  nearer  the  zenith.     Castor 
and  Pollux  will  lie  between  Orion  and  the  zenith,  and 
Capella  between  Aldebaran  and  Polaris.     The  western 
heavens  have  therefore  already  been  described.     Just 
east  of  the  meridian,  to  the  south  of  the  zenith,  is  the 
constellation   Leo.      The  most  conspicuous  group  of 
stars  is  the  Sickle,  in  the  head  of  the  Lion.     At  the 
end  of  the  handle  is  Regulus,  a  first-magnitude  star, 
but  not  a  very  bright  one.     East  of  the  Sickle  about 
16°  is  ft  Leonis. 

Corvus  is  low  down  in  the  southeast.  It  contains 
four  stars  of  medium  brightness,  forming  an  irregular 
four-sided  figure. 

Hydra  stretches  from  Procyon  across  south  of  the 
zenith  all  the  way  to  the  southeastern  horizon.  One 
second-magnitude  star,  «  Hydrse,  is  nearly  on  the 
meridian. 

Coma  Berenices  (Berenice's  Hair)  is  a  faint  cluster 
readily  seen  by  the  naked  eye,  following  Leo  up  the 
sky.  Between  it  and  the  horizon  is  the  brilliant  Arc- 
tu'rus  in  Bootes. 

Virgo,  containing  the  brilliant  Spica,  is  near  the 
horizon,  southward  from  Bootes. 

This  also  describes  the  heavens  at  10.30  o'clock  on 
March  1,  at  12.30  on  February  1,  and  so  on. 

219.  Southern  Constellations  Visible  in  Summer. — The 
Milky  Way  extends  from  the  north  to  nearly  the  south 
point  of  the  horizon  on  the  east  side  of  the  zenith. 

Of  the  constellations  previously  described,  Leo  will 


THE  CONSTELLATIONS.  235 


be  near  the  western  horizon,  and  Coma  Berenices  just 
above  it.  Virgo  will  be  low  in  the  southwest.  Bootes 
will  be  on  the  meridian,  with  Arcturus  southwest  of 
the  zenith. 

Corona,  a  semicircle  of  stars,  of  which  « is  of  the  second 
magnitude,  is  on  the  meridian,  nearly  in  the  zenith. 

Hercules  is  a  large  constellation  not  containing  any 
very  bright  stars,  which  adjoins  Corona  on  the  east. 

Lyra,  east  of  Hercules,  is  a  small  but  interesting  con- 
stellation. The  brightest  star  is  Vega  (a  Lyrae),  a  star 
of  the  first  magnitude.  Two  fainter  stars  form  with  it 
an  equilateral  triangle.  The  one  of  these  nearest  the 
pole  is  £  Lyrse,  a  quadruple  star  further  described  on 
page  251.  The  other  star,  £,  is  double.  South  of  these 
are  ft  and  y,  between  which  lies  the  ring  nebula  de- 
scribed on  page  253.  ft  is  a  variable  star  (see  page  256). 

Ophiuchus  lies  south  of  Hercules,  and  contains  noth- 
ing of  special  interest. 

South  of  this,  again,  is  Scorpio.  The  bright  star  in  it 
is  Antar'es.  It  may  be  known  by  its  ruddy  color  and 
by  the  two  stars  of  the  third  magnitude  between  which 
it  is  set. 

Cygnus,  between  Lyra  and  the  east,  contains  the 
"Cross."  It  lies  exactly  in  the  Milky  Way,  with  the 
long  arm  extending  in  its  direction.  At  the  head  of 
the  cross  is  Deneb  (a  Cygni). 

Aquila,  joining  Cygnus  on  the  south,  contains  Altair, 
of  the  first  magnitude.  It  lies  between  two  third-mag- 
nitude stars,  the  row  pointing  nearly  north  and  south. 

Delphinus  contains  "  Job's  Coffin,"  a  little  diamond 
of  stars  near  the  eastern  horizon. 

The  above  description  is  also  applicable  to  10.30 
o'clock  on  June  1 ;  12.30  on  May  1 ;  and  so  on. 


236  ASTRONOMY. 


220.  Southern  Constellations  Visible  in  Autumn. — The 
Dipper  is  now  near  the  northern  horizon.  The  Milky 
Way  stretches  through  the  zenith  from  the  northeast 
to  the  southwest. 

All  the  important  constellations  having  now  been 
described,  a  rapid  review  of  their  positions  is  all  that 
is  necessary. 

Cygnus  is  in  the  zenith,  and  Lyra  just  west  of  it. 
Delphinus  is  a  little  west  of  the  meridian,  to  the  south 
of  the  zenith,  and  Aquila  joins  it  to  the  southwest. 
Hercules  and  Corona  are  near  the  western  horizon.  To 
the  east  the  most  conspicuous  object  is  the  "  Square  of 
Pegasus,"  one  of  the  angles  of  which  is  a  Andromedse. 
Pisces  joins  Pegasus  on  the  east.  The  bright  star  Fo- 
malhaut,  in  Piscis  Australia,  is  low  down  in  the  south- 
southeast. 

This  also  describes  the  position  of  the  heavens  at 
10.30  o'clock  on  September  1 ;  12.30  on  August  1 ;  and 
so  on.1 

The  zodiacal  constellations  can  be  found  on  the 
maps.  Their  names  have  been  given  on  page  41. 
The  moon  and  all  the  planets  will  always  be  found  in 
these  constellations.  The  position  of  the  planets  should 
be  ascertained  beforehand,  as,  being  conspicuous  ob- 
jects, the  observer  may  lose  time  in  searching  for  them 
on  the  charts  if  he  mistake  them  for  stars. 

1  Any  one  who  desires  more  familiarity  with  the  heavens  than  can  be 
gained  from  this  general  sketch  should  study  them  with  the  aid  of  a 
large  star  atlas.  Heis's  Star  Atlas  is  convenient  and  reliable ;  Proc- 
tor's small  Star  Atlas  is  much  cheaper.  A  Planisphere  can  be  set  so 
as  to  show  the  positions  of  all  the  stars  with  reference  to  the  horizon 
at  any  time  of  night.  An  Astral  Lantern  is  a  cubical  box  with  glass 
sides,  within  which  is  a  lamp.  The  stars,  with  their  names  and  mag- 
nitudes, show  by  transmitted  light.  It  can  also  be  set  for  any  minute, 
and  the  names  easily  read  in  the  dark. 


23!) 


240 


1.J 


•4 
•4 


21 


241 


21* 


245 


1.-8. 


•I 
•  I 


246 


•I 
•I 

*4 
•I 
•I 


248 


DOUBLE  STARS.  249 


CHAPTER   II. 

DOUBLE    STAKS. — VARIABLE    STARS. — CLUSTERS    AND   NEB- 
ULA.— STRUCTURE    OF    THE    UNIVERSE. 

Double  Stars. 

221.  MANY  of  the  stars,  when  examined  with  a  tele- 
scope, are  seen  to  be  made  up  of  two  or  more  parts, 
which  are  so  close  together  that  they  present  to  the  eye 
the  appearance  of  a  single  star.  Sometimes  the  com- 
ponents are  of  nearly  equal  size,  and  sometimes  one  is 
so  faint  as  to  be  seen  only  with  large  telescopes.  An 
instance  of  the  former  is  Castor,  and  among  the  latter 
are  Sirius,  Rigd,  and  Polaris.  Sir  William  Herschel, 
who  first  carefully  studied  these  "  double  stars,"  at  first 
supposed  that  they  happened  to  be  nearly  in  the  same 
line  of  sight,  though  one  might  be  much  nearer  to  us 
than  the  other.  On  this  supposition  he  measured  their 
distance  and  direction  from  each  other,  hoping  that  the 
motion  of  the  earth  in  its  orbit  would  cause  an  apparent 
change  in  their  distance  apart,  and  that  thus  he  could 
determine  the  parallax  of  the  nearest  one.  After  he 
had  worked  at  this  for  some  time  he  became  aware 
that  there  was  in  some  cases  a  connection  between  the 
two  stars  entirely  different  from  what  he  had  expected. 
Their  distance  and  direction  from  each  other  changed 
too  rapidly  to  be  attributed  to  the  motion  of  the  earth 
alone.  He  finally  concluded  that  the  two  components 


ASTRONOMY. 


of  a  double  star  in  many  cases  were  revolving  about 
each  other.  This  has  been  established  beyond  doubt 
by  the  researches  of  other  observers.  The  two  or  more 
stars  constituting  such  a  group  are,  then,  members  of  a 
common  system.  They  revolve  about  their  centre  of 
gravity,  just  as  do  the  sun  and  the  earth,  or  the  earth 
and  the  moon  (Art.  113).  The  force  of  attraction  exists 
among  them,  and  their  motions  are  performed  in  obe- 
dience to  it.  Several  thousands  of  double  stars  are 
now  known ;  and  hundreds  are  added  to  the  list  every 
year.  A  list  of  such  as  are  the  most  easily  observed 
by  the  possessors  of  small  telescopes  is  given  in  Ap- 
pendix YI. 

Those  double  stars  in  which  a  motion  of  revolution 
about  each  other  has  been  certainly  seen  are  called 
binary  stars.  "When  there  are  three  or  more  in  the 
group,  they  are  called  triple  or  multiple  stars.  When 
they  are  in  the  same  line  of  sight  without  being  bina- 
ries, they  are  said  to  be  optically  connected.  New  ob- 
servations are  continually  placing  in  the  list  of  binary 
stars  those  which  were  previously  only  known  to  be 
optically  connected.  It  is  possible,  after  the  observa- 
tions of  several  years,  to  calculate  how  large  an  orbit 
one  of  a  pair  of  binary  stars  has,  and  how  long  it  will 
take  to  complete  it.  The  -period  of  revolution  for  those 
which  are  determined  the  most  accurately  varies  from 
twenty-five  to  one  thousand  years.  So  important  is  the 
measurement  of  double  stars  considered  to  be  that  some 
observatories  give  almost  their  whole  attention  to  it. 
In  the  course  of  a  series  of  years  enough  data  will  have 
accumulated  to  enable  us  to  find  the  orbits  of  many 
more,  and  thus  we  shall  gain  additional  knowledge  of 
the  condition  of  those  distant  suns. 


DOUBLE  STARS.  251 


Fig.  55   shows  the  orbit  which  one  of  the  compo- 
nents of  f  Virginis  makes  about  the  other.     The  num- 
bers on  the  diagram  indicate  the  years  when  the  stars 
were  in  that  relative  position.     It 
will  be  noticed  that  nearly  a  com- 
plete revolution  has  been  observed: 
at  one  end  of  the  line  we  have  the 
position   of   one    of  the   stars   in 
1718,  at  the  other  its  present  posi- 
tion. 

Figs.  56  to  59    show   some    of 

ffIB 

these  multiple  stars. 

FIG.  55.— ORBIT  OF  ?  VIRGINIS. 

When  more  than  two  stars  ex- 
ist in  the  group,  they  may  still  be  members  of  the  same 
system  :  thus,  of  the  stars  of  e  Lyrse,  shown  in  Fig.  67, 
the  two  nearest  each  other  perform  their  revolution 
in  about  one  thousand  years,  the  other  two  in  about 
two  thousand  years ;  and  each  pair  may  revolve  about 
the  other  in  an  orbit  of  immense  size  and  in  a  period 
which  is  many  thousands  of  years  long.  This  star 
can  be  seen  double  by  a  good  eye,  and  an  opera-glass 
shows  it  easily.  The  components  are  not  separable 
except  by  a  telescope. 

To  measure  double  stars  it  is  necessary  to  have  an 
instrument  attached  to  the  eye  end  of  the  telescope, 
called  a  micrometer.  This  will  be  explained  in,  a  subse- 
quent chapter.  The  two  measures  taken  are  the  dis- 
tance apart  of  the  components,  in  seconds  of  arc,  and 
the  angle  which  a  line  joining  them  makes  with  the 
meridian  through  the  brightest  one.  This  angle  is 
called  the  position  angle,  and  is  read  all  the  way  round 
from  the  north  by  the  east  from  0°  to  360°.  If  the  faint 
star  were  exactly  north  of  the  bright  one,  the  position 


252 


ASTRONOMY. 


angle  would  be  0°.  If  it  moved  eastward  around  the 
bright  one,  the  position  angle  would  increase ;  when 
south,  the  angle  would  be  180°,  and  when  west,  270°. 
It  is  clear  that  if  they  revolve  about  each  other,  one  of 
these  measures,  or  both,  must  change  in  course  of  time. 


£Cancri. 
(1865.)    Mags.  7, 


6  Orionis. 
Mags.  8, 12,  7&  6, 14,  7. 

FIGS.  56  TO  59.— MULTIPLE  STAES. 


e  Lyrse. 


If  we  are  looking  at  the  orbit  edgewise,  one  star  will 
seem  to  move  backward  and  forward  over  the  other, 
and  only  the  distance  will  vary ;  if  we  are  in  any  other 
position  with  respect  to  the  orbit,  the  angle  with  the 
meridian  will  also  vary. 


VARIABLE  AND  NEW  STARS.  253 

The  double  stars  are  sometimes  beautifully  colored, 
and  when  such  is  the  case  the  colors  are  usually  com- 
plementary1 to  each  other.  The  larger  star  is  most 
frequently  red  or  orange,  and  the  smaller  one  blue  or 
green.  Though  the  stars  are  in  some  instances  really 
colored,  it  is  probable  that  the  complementary  tints 
are  mainly  the  result  of  contrast. 

Some  stars  show  by  their  motions  that  they  are  at- 
tracted by  bodies  which  are  invisible.  These  are  dark 
worlds,  planets  perhaps,  but  often  must  bear  a  consid- 
erable ratio  to  the  mass  of  the  star.  The  bright  star 
Sirius  was  known  by  its  motions  to  have  a  "  compan- 
ion "  long  before  it  was  found.  It  was  at  last  discov- 
ered by  Clark,  the  telescope-maker  of  Cambridge, 
Mass.,  in  1862.  This  is  not  a  perfectly  dark  world, 
for  it  gives  about  10}o0  as  much  light  as  Sirius,  but  it 
must  weigh,  in  order  to  produce  the  disturbance,  about 
one-fourth  as  much. 


Variable  and  New  Stars. 

Nearly  all  the  stars  appear  to  remain  of  the  same 
brightness  night  after  night  and  year  after  year. 
A  few  of  them,  however,  are  perceptibly  brighter  at 
some  times  than  at  others.  These  are  called  variable 
stars.  We  will  describe  some  of  the  most  conspicuous 
of  them. 


1  Colors  are  complementary  when  their  union  produces  white. 
When  the  eye  notices  any  color,  and  is  then  quickly  turned  to  & 
white  or  nearly  white  object,  this  object  appears  of  a  color  comple- 
mentary to  the  first.  The  complementary  color  is  in  this  case  ao 
optical  illusion. 


22 


254  ASTRONOMY. 


222.  Algol. — This  star  is  marked  /?  in  the  constellation 
Perseus.     In  the  autumn  Perseus  is  in  the  northeast 
during  the  evening,  in  the  winter  nearly  overhead,  and 
in  the  spring  in  the  northwest.     Algol  may  be  found 
by  continuing  the  line  joining  Bigel  with  the  Pleiades 
half  as  far  beyond  the  latter.     Usually  its  magnitude 
is  2J.     For  two  and  one-half  days  it  continues  of  this 
magnitude  without  apparent  change;    then  for  four 
hours  it  fades  away,  till  it  becomes  of  the  fourth  mag- 
nitude ;  here  it  remains  for  about  twenty  minutes,  and 
then  through  four  hours  more  gradually  recovers  its 
original  brilliancy.     The  exact  period  from  one  mini- 
mum to  the  next  is  2  days,  20  hours,  and  49  minutes. 
Some  of  these  minima  occur  in  the  night,  and  some 
when  Algol  is  below  the  horizon.     If  the  student  can 
observe  one,  he  can  readily  count  forwards  so  as  to 
know  when  to  look  for  others. 

223.  Mira. — This  star  is  «  of  the  constellation  Cetus. 
It  comes  to  the  meridian  about  fifty  minutes  earlier 
than  Algol,  and  three  and  one-half  degrees  south  of  the 
equator.     It  is  sometimes  of  the  eleventh  magnitude,  so 
that  it  cannot  then  be  seen  by  the  naked  eye.     It  grad- 
ally  increases  from  this  to  the  second  magnitude.    After 
first  becoming  visible  it  requires  about  forty  days  to 
reach  its  maximum,  then  it  fades  out  of  sight  in  about 
two  months,  and  remains  invisible  for  seven  and  one- 
half  months,  thus  passing  through  all  its  changes  in 
about  eleven  months.     This  time  is  variable,  so  that 
its  return  cannot  be  exactly  predicted.     Its  maximum 
brightness  is  also  variable :  sometimes  it  is  almost  a 
first-magnitude  star ;  sometimes  at  its  brightest  it  does 
not  exceed  a  fourth.     It  is  expected  to  attain  its  great- 
est brilliancy  in  June,  1882,  May,  1883,  and  so  on ;  but, 


VARIABLE  AND  NEW  iSTAKS.  255 

as  the  period  varies  from  ten  to  twelve  months,  there 
may  be  some  deviation  from  this. 

224.  y  Argus. — This  star  is  in  the  Southern  hemi- 
sphere, and  can  never  be  seen  in  latitude  north  of  31° 
north.  Its  variations  of  brightness  are  greater  than 
those  of  any  other  periodic  star.  It  goes  through  its 
changes  in  a  period  of  seventy  years :  from  a  star  in- 
visible to  the  naked  eye  it  increases  almost  to  the 
brightness  of  Sirius.  Its  increase  is  not  uniform,  but 
numerous  small  fluctuations  may  be  noticed  along  its 
course.  The  curved  line  of  Fig.  60  shows  its  changes. 
The  horizontal  lines  indicate  magnitudes,  and  the  ver- 
tical lines  periods  of  ten  years.  Every  seventy  years  it 
goes  down  to  the  sixth  magnitude.  At  intervening  times 
its  brightness  varies  as  indicated  by  the  irregular  line. 


FIQ.  60. 


There  are  about  three  hundred  known  variables. 
The  following  table  gives  the  stars  in  which  the 
variations  may  be  most  easily  distinguished.  Those 
of  the  fifth  magnitude  and  over  can  be  seen  and  studied 
by  the  eye,  for  those  from  the  fifth  to  the  seventh  mag- 
nitude an  opera-glass  may  be  employed,  while  those 
below  the  tenth  magnitude  can  be  followed  only  by 
the  most  powerful  telescopes : 


256 


ASTRONOMY. 


Name. 

Variation  in 
Magnitude. 

Period. 

3  Librae  .... 

5     to    6 

?Persei  (Algol)  ... 

24  to   4 

Cephei  

3  7  to    48 

„ 

rj  Aquilaa  

34  to   6 

tt 

/3  Lyras        

31  to    44 

13         " 

5     to  12 

406         " 

R  Auriga;  .                     . 

6     to  13 

465         " 

o  Ceti 

2     to  11 

1     to    6 

Many  stars  are  known  to  vary  their  brightness  slightly 
without  any  regular  period  having  been  discovered. 
Thus,  Betelgeuse  («  Orionis)  is  sometimes  more,  but 
usually  less,  bright  than  Rigel  (/?  Orionis). 

225.  Cause  of  the  Variation.  —We  do  not  certainly  know 
the  cause  of  the  changes  of  brightness  of  variable  stars. 
There  may  be  a  dark  body  like  a  planet  revolving 
around  the  star,  which,  whenever  it  passes  in  front  of 
it,  cuts  off  a  portion  of  its  light.  This  is  the  theory 
which  best  explains  the  variations  of  such  stars  as 
Algol,  which  remain  of  the  same  brilliancy  during  most 
of  the  time,  and  at  regular  periods  become  fainter  for 
a  short  interval.  Other  variables,  which  are  gradually 
changing  from  one  extreme  to  the  other,  are  probably 
undergoing  a  real  variation  of  brightness  on  their  sur- 
faces. We  have  already  seen  that  the  sun  is,  at  regu- 
lar intervals  of  about  eleven  years,  largely  covered  with 
spots.  These  spots  may  slightly  diminish  its  bright- 
ness, so  that  it  can  be  considered  a  variable  star  with  a 
period  of  about  eleven  years.  If  we  suppose  the  spots 
to  be  greatly  increased  in  number,  so  as  largely  to  dim 
the  surface  of  the  sun,  and  this  dimness  to  occur  at 
regular  periods,  the  phenomena  of  variable  stars,  as  we 
notice  some  of  them,  would  be  accounted  for. 


VARIABLE  AND  NEW  STARS.  257 

226.  How  to  observe  Variable  Stars. — The  method  de- 
vised by  Argelander1  of  making  observations  on  vari- 
able stars  is  as  follows.  Begin  the  watch  a  half-hour 
or  more  before  the  star  will  begin  to  change,  and  select 
two  stars  near  the  variable,  one  a  little  brighter  and 
the  other  a  little  fainter.  Now,  if  the  difference  between 
the  brighter  one  and  the  variable  is  so  slight  that  we 
could  not  imagine  a  star  between  the  two,  then  the 
first  star  is  said  to  be  one  step  brighter  than  the  varia- 
ble ;  and  if  the  variable  is  so  much  brighter  than  the 
other  that  just  one  star  could  be  imagined  between 
them,  the  variable  would  be  brighter  by  two  steps.  If 
two  stars  could  be  supposed  inserted  between  them, 
there  is  a  difference  of  three  steps ;  and  so  on.  Calling 
the  comparison  stars  a  and  6,  and  the  variable  v,  the 
observation  is  noted  thus : 

1880,  May  5,  10.15  P.M.,  alvZb. 

This  means  that  a  is  one  step  brighter  than  v,  and  v 
two  steps  brighter  than  b.  If  the  star  is  one  that 
changes  rapidly,  as  Algol,  it  should  be  observed  every 
few  minutes  until  its  changes  are  over ;  if  it  changes 
slowly,  as  Mira,  once  a  day. is  often  enough.  The  next 
observation  of  our  star  may  be  this : 

10.35  P.M.,  a  2  v  1  b; 
and  the  next : 

11  P.M.,  v  =  b. 
If  the  star  grow  still  fainter,  a  new  and  fainter  com- 

1  Argelander,  1799-1875.     In  charge  of  the  Observatory  of  Bonn, 
Prussia,  „.  # 


258  ASTRONOMY. 


parison  star  should  be  taken,  and  the  observations  may 
go  on,  thus : 

11.30  P,M.,  blv4c; 

11.55  P.M.,  b  2v  3  c; 

May  6,  12.20  A.M.,  b  3  v  2  c; 

12.30  A.M.,  b  3-4  v  1-2  c; 
12.40A.M.,  6  3  v  2c; 
1       A.M.,  b  2  v  3  c\ 
1.30  A.M.,  6  1  v  4  c. 

Here  we  see  that  the  star  was  at  its  minimum  about 
half-past  twelve,  when  it  was  three  or  four  steps  fainter 
than  b  and  one  or  two  steps  brighter  than  c.  The 
observations  should  be  continued  till  the  star  has  come 
back  to  its  usual  magnitude.  If  the  exact  magnitudes 
of  the  comparison  stars  are  found  from  a  catalogue 
or  elsewhere,  the  amount  of  change  that  the  variable 
makes,  as  well  as  the  time  of  minimum1  or  maximum, 
will  also  become  known  from  the  observations.  The 
changes  are  frequently  irregular,  and  these  may  be 
made  more  striking  by  laying  them  off  in  curves,  as 
in  Fig.  60,  which  shows  the  light-curve  of  ^  Argus. 
The  higher  parts  of  the  curve  show  when  the  star  is 
brightest;  we  can  see  that  it  varies  from  the  first 
to  the  sixth  magnitude  in  seventy  years,  but  that  it 
changes  irregularly. 

It  must  be  remembered  that  a  step  is  the  least  possi- 
ble difference,  and  that  if  there  is  room  for  an  interme- 
diate star  the  difference  is  two  steps.  A  step  has  been 
found  to  be  in  practice  about  one-tenth  of  a  magnitude. 

1  Minimum  here  means  the  least  brightness,  and  maximum  the 
greatest. 


VARIABLE  AND  NEW  STARS.  259 

One  should  not  trust  himself  to  estimate  a  difference 
of  more  than  four  steps,  but  should  use  another  closer 
comparison  star.  The  condition  of  the  sky,  as  clear, 
hazy,  moonlight,  etc.,  should  he  noted  with  the  obser- 
vation. 

227.  New  Stars. — New  or  temporary  stars  are  such  as 
suddenly  blaze  out  and  shortly  afterwards  disappear. 
They  differ  from  variable  stars  in  that  their  increase 
of  brightness  is  more  striking  and  does  not  return  at 
regular  periods.  The  most  noted  of  these  was  seen 
by  Tycho  Brahe1  in  1572.  He  noticed  a  star  of  the 
first  magnitude  where  he  was  certain  it  had  not  existed 
a  half-hour  before.  It  continued  to  increase  till  it 
exceeded  any  other  star  in  the  heavens  and  could  be 
seen  at  mid-day ;  then  it  gradually  faded  away  till  it 
vanished  altogether. 

There  are  several  other  cases  of  new  stars  on  record. 
One  of  these,  which  appeared  in  1866  and  increased  till 
it  became  of  the  second  magnitude,  was  examined  by 
the  spectroscope:  it  was  found  that  the  most  of  the 
light  was  due  to  the  presence  of  hydrogen  gas  so  heated 
as  to  cause  the  great  brilliancy.  We  have  seen  that 
the  red  prominences  in  the  sun  are  composed  of  the 
same  gas.  Hence  we  are  led  to  infer  that  the  cause  of 
the  sudden  brilliancy  of  the  star  was  a  great  outrush 

1  A  Danish  astronomer,  1545  to  1601.  "  As  a  practical  astrono- 
mer," says  Sir  David  Brewster,  u  Tycho  has  not  been  surpassed  by 
any  observer  of  ancient  or  modern  times.  The  splendor  and  number 
of  his  instruments,  the  ingenuity  which  he  exhibited  in  inventing 
new  ones,  and  his  skill  and  assiduity  as  an  observer,  have  given  a 
character  to  his  labors  and  a  value  to  his  observations  which  will 
be  appreciated  to  the  latest  posterity."  He  rejected  the  Copernican 
theory  because  he  supposed  it  to  be  contrary  to  the  Bible. 


260  ASTRONOMY. 


of  burning  hydrogen,  which,  partly  by  its  own  light 
and  partly  by  heating  the  surface  of  the  star,  gave  rise 
to  the  unusual  brightness.  Such  an  outbreak  on  the 
sun  would  so  raise  its  temperature  that  life  on  the  earth 
would  instantly  be  destroyed.  A  single  case  is  not 
sufficiently  conclusive  to  prove  this  theory  for  all  new 
stars ;  at  all  events,  they  are  not  suddenly  created  out 
of  nothing,  as  was  formerly  supposed :  the  stars  existed 
previously,  and  the  brilliancy  was  the  result  of  some 
change  on  the  star  itself.  It  is  probable  that  some 
of  these  are  variable  stars  of  long  period,  and  that  we 
have  observed  only  the  one  maximum. 

In  1885  a  star  of  the  6th  magnitude  suddenly  ap- 
peared in  the  midst  of  the  Great  Nebula  of  Andromeda 
(see  page  266),  and  slowly  faded  away. 

Clusters  and  Nebulae. 

228.  Clusters. — An  observer  of  the  heavens  will  no- 
tice that  the  stars  are  not  uniformly  distributed,  but 
are  frequently  collected  into  dusters.  The  Pleiades  and 
Hyades  are  examples  of  these.  The  six  stars  of  the 
former  visible  to  an  ordinary  eye  become  transformed 
into  six  hundred  in  the  telescope.  Another  illustration 
is  Coma  Berenices,  which,  in  the  evening  through  the 
spring,  may  be  seen  following  Leo  around  the  heavens. 
A  careful  observer  will  also  notice  patches  of  misty 
light,  which  a  very  small  telescope  will  convert  into 
stars.  One  of  these,  which  may  be  seen  on  any  clear 
night,  is  the  "  Beehive  cluster"  in  Cancer.  It  is  about 
half-way  between  Regulus  and  Castor,  a  little  out  of  the 
line  joining  them.  Another  may  be  found  in  Perseus 


CLUSTERS  AND  NEBULAE.  261 

by  producing  the  line  joining  y  and  d  Cassiopeise  be- 
yond the  latter  a  distance  equal  to  twice  their  distance 
apart.  In  the  telescope  it  is  a  beautiful  mass  of  stars. 
Others  will  be  found  named  in  Appendix  VII. 


FIG.  61.— CUWTBK  m  AQUARIUS, 

Telescopes  show  a  large  number  of  other  clusters. 
Many  of  these  do  not  present  any  regular  form.  Some 
of  them  have  a  circular  outline,  with  the  stars  more 
closely  packed  near  the  centre.  The  inference  to  be 
drawn  from  this  appearance  is  that  the  cluster  is  in 
the  shape  of  a  globe ;  hence  we  should  look  through  a 
greater  stretch  of  stars  near  the  centre  than  around 
the  edges.  But  as  such  collections  may  constitute  sys- 
tems, held  together  by  central  attraction,  there  may 


262  ASTRONOMY. 


also  be  a  real  condensation  at  the  centre.  The  tele- 
scopic appearance  of  some  clusters  may  be  seen  in  the 
drawings  on  pages  261  and  262. 


*io.  62.— CLUSTER  IN  LIBRA.  Fio.  63.— CLUSTER  IN  HERCULIB. 

229.  Nebulce. — In  addition  to  the  clusters  there  may 
be  seen  in  the  telescope  masses  of  misty  light,  which 


Pio.  64.— SINGULAR  CLUSTERS. 


cannot  be    resolved   into    stars.      These   are  nebulae. 
Some  of  them  are  clusters  so  far  away  or  so  faint  that 


CLUSTERS  AND   NEBULA.  263 


Fio.  f>r>.— GREAT  NEBULA  OF  ORION. 


264  ASTRONOMY. 

the  telescopic  power  is  not  sufficient  to  resolve  them. 
It  was  thought  for  a  time  that  as  every  increase  of 
power  resolved  more  and  more  of  these  into  stars, 
when  telescopes  could  be  made  great  enough  they 
would  all  be  so  resolved.  But  the  spectroscope  is  able 
to  tell  from  the  character  of  the  light  whether  it  comes 
from  a  solid  or  a  gaseous  source :  when  directed  to  any 
of  the  resolvable  nebulae,  it,  like  the  telescope,  gives 
evidence  of  their  solid  condition ;  but  when  some  of 
the  others  are  studied  by  it,  it  shows  that  they  are  just 
what  they  seem  to  be, — masses  of  luminous  gas.  Such 
nebulae  are  not,  then,  collections  of  suns  so  far  away 
that  they  seem  to  be  clouds ;  they  are  entirely  different 
from  the  stars ;  yet  at  the  same  time  they  give  out  light 
of  their  own,  and  are  not  rendered  visible  by  reflecting 
the  light  of  the  suns  around.  They  may  be  gradually 
condensing  into  suns,  and  perhaps  serve  to  show  the 
early  stages  through  which  all  the  stars  have  passed. 
In  the  course  of  a  long  period  of  time  a  sun  may  be 
formed  from  each  of  them. 

230.  Classification. — There  are  over  eight  thousand 
known  nebulae  in  the  heavens.  They  are  divided  ac- 
cording to  their  form  into  five  classes, — irregular,  an- 
nular or  elliptical,  spiral,  planetary,  and  nebulous  stars. 
The  first  are  the  most  common.  The  "  Great  Nebula 
of  Orion"  is  an  example.  It  may  be  seen  with  the 
naked  eye  on  a  moonless  night  as  a  faint  light  sur- 
rounding the  star  0,  the  middle  one  of  the  three  in  the 
sword.  The  star  itself  is  a  multiple  star,  the  four 
brightest  components  of  which  constitute  what  is  called 
the  trapezium1  of  Orion  (see  Fig.  57).  From  around 

1  A  trapezium  is  a  four-sided,  irregular  figure. 


AND  NEBULA. 


265 


if «*«*-*  of  ANDROMEDA. 


266  ASTRONOMY. 


these  the  nebula  stretches  out  in  irregular  bands  and 
patches,  as  shown  in  the  drawing  on  the  preceding 
page.  It  envelops  several  of  the  neighboring  stars, 
and  seems  in  an  indefinable  manner  to  cover  the  whole 
region  thereabouts. 

Ring  nebulae  are  quite  rare.  One  in  Lyra  is  shown 
in  1  and  2,  Fig.  66.  It  is  situated  about  midway  be- 
tween the  stars  /9  and  ^,  and  can  be  seen  with  a  small 
telescope.  Sir  John  Herschel  says,  "  The  central 
vacuity  is  not  quite  dark,  but  is  filled  in  with  faint 
nebulae,  like  a  gauze  stretched  over  a  hoop."  The 
other  drawings  of  Fig.  66  show  other  ring  nebulae. 
Quite  recently  a  ring  40'  in  diameter,  but  very 
faint,  has  been  discovered  in  the  constellation  Mo- 
noceros.  This  is  interesting  from  its  relatively  large 
size. 

Elliptic  nebulae  are  classed  with  these,  because  they 
may  be  the  same  seen  edgewise.  The  most  conspicu- 
ous of  these  is  the  "Great  Nebula  of  Andromeda," 
which  is  situated  not  far  from  ft  Andromedae.  It  looks 
to  the  naked  eye  like  a  mass  of  diffused  light,  and 
has  often  been  mistaken  for  a  comet.  The  spectro- 
scope seems  to  indicate  that  it  is  not  gaseous,  though 
the  most  powerful  telescope  fails  to  resolve  it  into 
stars.  Fig.  67  shows  the  appearance  of  this  nebula 
as  seen  in  a  large  telescope.  Recently  it  has  been 
photographed  by  an  exposure  of  about  two  hours, 
and  the  picture,  as  seen  in  Fig.  68,  presents  an  en- 
tirely different  appearance.  The  annular  form  is 
clearly  seen,  and  the  dark  rifts  are  evidently  gaps 
between  the  rings  like  the  similar  forms  in  the  case 
of  Saturn. 

The  camera  not  only  gives  us   better  pictures  of 


CLUSTERS  AND  NEBULA. 


267 


FIG.  68. — THE  NEBULA  IN  ANDROMEDA. 
(Photographed  by  Mr.  Roberts,  December  30,  1888.) 

known  objects,  but  it  also  brings  to  light  a  number  of 
new  nebulae.  The  region  around  the  Pleiades  seems 
to  be  full  of  nebulae,  and  many  of  the  stars  of  this  clus- 


268  ASTRONOMY. 


ter  have  nebulous  wisps  extending  from  them.  These 
can  be  detected  only  on  the  photographic  plates  and 
not  by  direct  observation.  Fig.  69  shows  several  cir- 
cular and  elliptic  nebulae. 


FIG.  69.— CIRCULAR  AND  ELLIPTIC  NEBULA. 


Spiral  nebulae  can  be  seen  as  such  only  in  the  largest 
telescopes.  Fig.  70  represents  one  as  it  appeared  in 
Lord  Rosse's  great  reflector. 

Planetary  nebulae  are  so  called  because  they  resemble 
a  planet  in  appearance.  In  the  telescope  a  star  looks 
like  a  point  of  light,  brighter  but  no  larger  than  when 


CLUSTERS  AND  NEBULA.  269 

viewed  with  the  eye  alone.     A  planet,  however,  has  its 
disk  magnified  when  viewed  in  the  telescope,  so  as  to 


FIG.  70.— SPIBAL  NEBULA. 


appear  of  appreciable  size.      A  planetary  nebula  is 
uniformly  bright,  and  often  has  a  well-defined  outline, 


270  ASTRONOMY. 


so  that  it  might  be  mistaken  for  one  of  the  outer  planets 
of  the  solar  system. 

Nebulous  stars  are  so  named  because  they  seem  to 
be  surrounded  by  an  ill-defined  nebulous  atmosphere. 
It  is  noticed  in  a  few  cases  of  elliptic  nebulae  that  stars 


FIG.  71.— DUMB-BELL  NEBULA  IN  VULPECULA. 

occupy  positions  near  the  two  foci l  of  the  ellipse.     In 
other  cases  the  stars  seem  to  be  in  the  centre  of  a  neb- 

1  See  page  33. 


CLUSTERS  AND  NEBULA. 


271 


ulous  mass.  These  arrangements  are  too  common  to 
be  the  results  of  chance,  and  it  is  probable  that  there 
is  some  physical  connection  between  the  stars  and  neb- 
ulae. Such  stars  are  often  variable.  A  very  conspicu- 
ous nebula  surrounds  the  remarkable  variable,  ^  Argus. 
231.  Magellanic1  Clouds. — These  are  two  nebulous 
objects  which  can  be  seen  by  the  naked  eye  in  the 
Southern  hemisphere.  When  examined  with  a  tele- 
scope they  are  shown  to  be  made  up  of  a  collection  of 


Fia.  72.  —  DOUBLE 


nebulse,  clusters,  and  single  stars,  crowded  together  in 
great  confusion,  —  a  kind  of  miniature  sidereal  system. 
232.  Variable  Nebulce.  —  There  are  nebulae,  like  stars, 
which  vary  in  brightness  at  different  times.  !N"ew  neb- 
ulae have  also  been  announced  where  none  had  been 
known  to  exist  previously.  We  have  likewise  the  phe- 

1  Named  from  Magellan,  the  navigator. 


272  ASTRONOMY. 


nomena  of  double  nebulae,  the  parts  of  whicb  may 
revolve  about  each  other. 


Structure  of  the  Universe. 

233.  The  greatest  problem  which  astronomers  have 
ever  attempted  to  solve  is  the  determination  of  the 
shape  and  structure  of  the  sidereal  universe  taken  as 
a  whole.     We  do  not  know  that  we  have  reached  the 
outer  bounds  of  the  solar  system ;  there  may  be  planets 
outside  the  orbit  of  Neptune ;  there  are  probably  an 
immense  number  of  planetoids  and  meteors,  of  which 
we  know  nothing,  inside  its  orbit.      Since  our  infor- 
mation is  so  imperfect  concerning  the  construction  of 
the  system  in  which  we  are,  it  might  be  expected  that 
anything  regarding  the  bounds  of  the  great  sidereal 
universe  would  be  out  of  our  reach.     In  most  portions 
of  the  heavens  the  only  effect  of  more  powerful  tele- 
scopes is  to  bring  into  view  more  stars  and  nebulae, 
without  seeming  to  pierce  through  the  stratum  to  any 
vacuity  beyond.     The  largest  glasses  ever  constructed 
show  a  thousand  times  as  many  stars  as  we  see  by  the 
eye;  but  they  reveal  also  faint  glimmerings  of  light 
which  tell  of  clusters  beyond  their  reach.     If  the  light 
from  the  nearest  of  the  stars  is  years  on  its  way  to  us, 
the  light  from  some  of  these  outlying  members  has 
been  coming  to  us  for  centuries.     These  facts  suggest 
to  us  numbers  which  we  cannot  even  imagine ;  the  dis- 
tance to  the  boundaries  of  the  universe  is  inconceivable, 
and  to  tell  anything  of  its  shape  or  its  structure  may 
well  seem  a  hopeless  problem. 

234.  Distribution  of  the  Stars. — There  are,  however,  a 
few  facts  which  throw  a  little  light  on  the  question. 


STRUCTURE   OF  THE   UNIVERSE.  273 

Sir  William  Herschel,  in  order  to  aid  in  its  solution, 
undertook  a  system  of  "  star-gauging."  This  consisted 
in  systematically  going  over  the  heavens,  pointing  his 
telescope  to  every  part,  and  counting  the  number  of 
stars  in  the  field  of  view.  By  this  means  he  found  that 
they  were  not  distributed  uniformly  over  the  sky,  but 
were  arranged  with  some  regularity  with  reference  to 
the  Milky  Way.  He  found  that  the  nearer  to  the 
Milky  Way  his  telescope  pointed,  the  greater  was  the 
number  of  stars  he  could  count  in  its  field  of  view  at 
any  one  time ;  and  that  the  place  in  the  heavens  most 
barren  of  stars  was  the  region  that  surrounded  the  poles 
of  the  Milky  Way, — the  points  just  90°  from  it.  Having 
ascertained  with  great  certainty  this  law  of  distribution, 
he  then  took  it  for  granted  that  the  stars  were  distrib- 
uted uniformly  through  space;  that  is,  that  they  were  all 
separated  from  one  another  by  equal  intervals.  When 
he  looked  into  his  telescope  and  counted  only  a  few 
stars,  the  inference  would  be  that  the  system  came  to  a 
limit  soon  in  that  direction ;  if  the  field  of  view  was 
crowded  with  stars,  it  might  be  expected  that  in  that 
direction  the  system  extended  to  a  great  distance,  star 
beyond  star,  each  star  separated  from  every  other  by  a 
distance  as  great  as  that  between  our  sun  and  its  nearest 
neighbor.  As  he  had  found  that  the  stars  were  strewn 
more  closely  as  he  approached  the  Milky  Way,  he  con- 
cluded that  the  universe  was  a  flattened,  lens-shaped 
mass,  having  its  greatest  extent  in  the  direction  of  the 
Milky  Way  :  when,  then,  we  look  in  that  direction  the 
ring  of  light  we  see  there  indicates  the  great  stretch  of 
the  universe  in  that  plane ;  when  we  look  at  right  angles 
to  this  plane  our  gaze  comparatively  soon  reaches  out 
beyond  its  limits.  This  theory  is  wholly  based  on  the 


274  ASTRONOMY. 


assumption  of  the  equal  distribution  of  the  stars ;  if  it 
he  true,  as  seems  probable,  that  the  stars  are  crowded 
more  closely  in  the  plane  of  the  Milky  Way  than  else- 
where, it  may  be  that  the  universe  is  no  more  extended 
in  that  direction  than  in  others.  But  whatever  be  the 
outline  of  the  universe,  considered  as  a  whole,  Her- 
schel's  investigations  undoubtedly  show  that  the  greater 
number  of  stars  are  clustered  near  the  plane  of  the 
Milky  Way,  and  that  we  are  situated  in  that  plane,  or 
near  to  it.  The  Milky  Way  may  be  such  a  flat  disk 
as  Herschel  describes,  or  it  may  be  a  ring ;  in  the  latter 
case  we  must  suppose  it  to  be  filled  inside  with  a  looser 
company  of  stars,  of  which  our  sun  is  one. 

235.  Distribution  of  the  Nebulce. — The  nebulae  are  ar- 
ranged very  differently  from  the  stars.     While  many 
clusters  are  in  and  near  the  Milky  Way,  the  real  ir- 
resolvable nebulae  are  there  distributed  least  profusely. 
The  constellation  which  contains  the  most  of  them  is 
Virgo,  which  is  situated  as  far  from  the  Milky  Way  as 
possible ;  on  all  sides  of  this  they  diminish  in  frequency 
with  considerable  regularity.     The  figure  on  the  oppo- 
site page,  drawn  by  Richard  A.  Proctor,  shows  the 
Milky  Way  and  the  distribution  of  the  nebulae.     Each 
dot  is  a  nebula.     It  will  be  seen  that  they  increase  in 
frequency  as  we  depart  from  the  Milky  Way. 

236.  The  Universe. — So  far  as  our  knowledge  of  the 
great  sidereal  system  extends,  which  is  only  a  very 
little  way,  we  may,  then,  consider  it  to  be  either  a  flat 
disk  or  ring  of  stars,  of  which  the  sun  is  one,  and  that 
its  greatest  extent  is  in  the  direction  of  the  Milky 
Way;  while  on  either  side  of  this  plane  are  groups  of 
nebulae,  interspersed  with  a  small  number  of  stars.     It 
is  a  very  great  and  complicated  universe.     The  stars  in 


276  ASTRONOMY. 


it  are  moving  in  all  conceivable  directions,  and,  so 
far  as  can  now  be  known,  do  not  revolve  about  any 
common  centre,  as  is  the  case  with  the  solar  system.1 
In  all  probability,  around  these  suns  are  moving  mul- 
titudes of  dark  worlds,  while  comets  are  speeding  in 
all  directions,  messengers  from  one  solar  system  to 
another.  All  is  regulated  by  material  laws,  which  keep 
every  member  in  its  place,  and  over  all  and  in  all  rules 
the  Great  Lawgiver. 

237.  The  Nebular  Hypothesis. — But  the  question 
comes  up,  What  has  been  the  past  history  of  the  uni- 
verse ?  Was  it  created  just  as  we  study  it  now  ?  This 
is  not  probable.  There  has  doubtless  been  a  gradual 
growth  to  its  present  condition.  Through  what  stages 
the  growth  has  been  carried  we  do  not  certainly  know. 
There  is  a  theory,  commonly  called  the  nebular  hypothe- 
sis, which  will  account  for  many  of  the  facts,  but  which 
seems  to  be  disproved  by  others.  As  it  has  received  a 
wide  notoriety,  we  will  explain  it  briefly  here.  The 
theory  is  that  every  star,  with  its  attendant  system  of 
worlds,  was  at  one  time  in  the  form  of  a  gaseous  neb- 
ula. A  motion  of  rotation  was  set  up  in  this  mass. 
The  central  attraction  would  tend  to  condense  it 
towards  the  centre ;  as  it  contracted  in  volume  its  ve- 
locity of  rotation  would  increase,  and  the  tendency  of 
the  parts  around  the  equator  of  the  mass  to  fly  out  from 
the  centre  would  also  increase.  Hence  there  would 
be  thrown  off  around  the  outer  edge  of  the  revolving 
nebula  a  ring  of  matter,  and  the  remainder  of  the  neb- 
ula would  go  on  contracting,  leaving  the  ring  separated 

1  A  theory  has  been  proposed  that  Alcyone,  the  brightest  of  the 
Pleiades,  is  the  centre  of  the  sidereal  system.  There  is  no  satisfactory 
proof  of  this,  and  astronomers  consider  ic  improbable. 


STRUCTURE   OF  THE   UNIVERSE. 


from  it.  When  the  contraction  went  on  farther,  a  sec- 
ond ring  would  be  thrown  off,  and  the  process  would 
go  on  till  the  central  mass  became  a  sun.  Fig.  74  is  a 
fanciful  picture  of  the  appearance  of  the  solar  system 
at  one  stage  of  development,  but  may  be  compared 
with  the  photograph  of  the  Andromeda  nebula  of 
Fig.  68. 

The  rings  which  had  been  thrown  off  at  various  times 
would  also  condense  by  radiating  heat  to  the  cooler  space 


FIG.  74. — ILLUSTRATION  OF  THE  NEBULAR  HYPOTHESIS. 


around  ;  if  the  condensation  was  about  equal  all  around 
the  ring,  a  number  of  small  masses  would  be  formed, 
and  the  phenomenon  of  our  ring  of  planetoids,  or  of  the 
rings  of  Saturn,  would  be  presented ;  if,  however,  one 
portion  were  denser  than  the  rest,  it  would  gradually 
attract  the  other  parts  to  it,  till  the  whole  ring  was 
joined  in  a  single  planetary  mass.  This  mass  might  in 

24 


278  ASTRONOMY. 


its  turn  condense  and  throw  off  rings  which  would  form 
the  satellites. 

We  can  trace  the  possible  development  further.  The 
planets  would  be  at  a  great  heat,  at  first  being  gaseous, 
and  then  liquid ;  in  course  of  time,  by  the  continual 
radiation  of  heat,  a  crust  would  be  formed  on  their 
surfaces  which  would  gradually  be  prepared  for  habi- 
tation; the  larger  bodies,  the  central  masses,  would 
cool  more  slowly,  and  thus  their  worlds  could  have  the 
benefit  of  their  light  and  heat;  on  the  other  hand,  the 
small  moons  would  soon  become  cold  and  barren,  as 
we  know  our  moon  to  be. 

The  facts  in  support  of  this  theory  are, — 

First.  In  our  solar  system  the  planets  all  revolve 
around  the  sun  in  one  direction,  and  nearly  in  the  same 
plane.  The  satellites  in  general  move  about  their  pri- 
maries in  the  same  direction,  and  nearly  in  the  same 
plane,  and  the  planets,  with  the  probable  exceptions  of 
Uranus  and  Neptune,  turn  on  their  axes  the  same  way. 
If  they  had  ever  been  parts  of  a  common  revolving 
body,  they  would  of  necessity  show  this  common 
direction  of  rotation. 

Second.  Matter  in  the  interior  of  the  earth  is  known 
to  be  in  a  liquid  molten  state.  The  heat  increases  as 
we  descend  into  the  earth,  and  the  effects  of  heat  are 
shown  in  the  igneous  and  metamorphic  rocks. 

Third.  We  see  in  the  heavens  a  number  of  nebulae 
which  seem  to  be  in  the  various  stages  of  development 
in  this  direction,  and  which  the  spectroscope  now  shows 
to  be  gaseous.  According  to  the  theory,  these  are,  then, 
systems  in  process  of  growth. 


PART   III. 

ASTRONOMICAL  INSTRUMENTS. 


238.  Properties  of  Light. — We  will  now  briefly  con- 
sider such  of  the  properties  of  light  as  are  necessary  to 
the  correct  understanding  of  the  principles  involved  in 
the  construction  of  the  telescope  and  the  spectroscope. 

When  a  body  is  luminous  it  is  so  on  account  of  a 
rapid  vibration  of  its  particles.  These  vibrations  are 
conveyed  by  the  ether.  This  ether  fills  up  all  the 
space  between  the  different  bodies  of  the  universe,  and 
also  exists  in  the  pores  of  matter ;  when  these  waves 
enter  the  eye  they  affect  the  nerve  and  brain  in  such  a 
way  as  to  give  us  the  sensation  of  light. 

Waves  of  light,  when  they  pass  through  a  substance 
of  uniform  density  and  transparency,  move  in  straight 
lines.  When  they  strike  a  smooth  surface  which  they 
cannot  penetrate,  they  are  reflected,  and  bound  off, 
making,  in  the  opposite  direction,  the  same  angle  with 
the  perpendicular  to  the  surface  which  they  had  before 
scriking.  As  we  see  an  object  by  means  of  the  rays 
of  light  which  pass  from  it  to  the  eye,  it  appears  to  be 
in  the  direction  from  which  the  light  comes  as  it  enters 
the  eye.  Thus,  in  a  mirror  the  contents  of  a  room  seem 
to  lie  behind  the  wall,  because  the  light  from  them, 
turned  back  by  the  mirror,  moves  from  that  direction. 

•  ,  279 


280  ASTRONOMY. 


When  these  waves  of  light  pass  from  one  medium  to 
another,  transparent  but  of  different  density,  they  do 
not  turn  back,  but  slightly  change  their  course.  This 
change  of  course  is  termed  refraction.  We  have  shown 
in  Chapter  IV.  the  effect  of  the  refraction  of  the  dif- 
ferent strata  of  the  atmosphere.  If  the  waves  pass  into 
a  piece  of  glass  at  an  angle,  the  same  phenomena  are 
noticed;  the  direction  changes  so  as  to  agree  more 
nearly  with  the  perpendicular  to  the  surface. 

Another  phenomenon  besides  refraction  takes  place 
when  light  passes  from  one  transparent  medium  into 
another.  The  waves  which  make  up  a  ray  of  ordinary 
light  are  of  different  lengths;  some  vibrate  rapidly, 
and  when  they  reach  the  eye  alone  give  the  sensation 
of  violet  light,  and  some  vibrate  more  slowly,  and  give 
the  idea  of  red ;  while  between  these  are  all  the  other 
colors  of  the  rainbow.  A  ray  of  ordinary  light  con- 
tains all  these  colors,  and  when  it  enters  obliquely  a 
transparent  medium  of  different  density  the  short  blue 
rays  are  turned  from  their  course  more  than  the  longer 
red  ones,  and  we  see  the  rainbow  colors.  This  is  called 
dispersion. 

239.  Velocity  of  Light. — The  fact  that  light  requires 
a  certain  time  to  pass  from  one  point  to  another  was 
discovered  by  Romer1  in  1675.  He  noticed  that  the 
times  of  the  eclipses  and  transits  of  Jupiter's  satellites 
occurred  later  when  the  earth  was  on  the  side  of  its 
orbit  opposite  to  Jupiter  than  when  it  was  nearer  to 
him.  The  first  one  of  these  points  is  farther  from  Ju- 
piter than  the  other  by  a  distance  equal  to  the  diameter 
of  the  orbit  of  the  earth.  Thus,  if  EE'  be  the  earth's 

1  Ro'mer,  a  Dane,  1644-1710. 


ASTRONOMICAL  INSTRUMENTS. 


281 


orbit,  S  the  sun,  and  J  the  position  of  Jupiter,  the  earth 

at  E  is  nearer  to  Jupiter  than  at  E'  by  the  distance 

EE',  the  diameter  of  its  orbit.     He 

therefore  rightly  concluded  that  the 

reason  of  the  lateness  was  the  greater 

distance  the  light  has  to  pass  over  in 

one  case  than  in  the  other.     This 

lateness  amounts   to   about  sixteen 

and  one-half  minutes.     The  time  it 

requires  light  to  pass  over  the  space 

which  separates  the  earth  from  the 

sun  thus  becomes  known,  and  from 

this,  if  we  know  the  velocity  of  light, 

we  can  determine  the  distance  to  the 

sun.    Very  careful  investigation  has 

shown  that  the  time  necessary  for 

light  to  pass  from  the  earth  to  the  sun  is  498  seconds. 

By  multiplying  498  by  the  number  of  miles  that  light 

moves  in  one  second,  we  obtain  the  distance  to  the  sun 

in  miles. 

It  therefore  becomes  a  very  important  problem  to 
determine  the  velocity  of  light.  Several  methods  have 
been  used,  which  are  described  in  treatises  on  natural 
philosophy.  The  one  which  has  produced  the  best  re- 
sults is  that  which  was  first  suggested  by  Foucault,1  and 
which  has  since  been  carried  to  a  great  degree  of  per- 
fection by  Michelson.2  The  outlines  of  the  method  are 
as  follows.  Sunlight  is  allowed  to  pass  through  a  nar- 
row slit  and  fall  on  a  mirror  which  is  rapidly  revolving; 
from  this  it  is  reflected  to  another  mirror,  which  turns 


FIG.  75. 


1  Foucault  (foo-ko'),  a  French  natural  philosopher,  1816-1868. 

2  Michelson,  Professor  at  U.  S.  Na   al  Academy  at  Annapolis. 

24* 


282  ASTRONOMY. 


it  back  to  the  revolving  mirror,  and  thence  to  the  slit. 
If  the  light  were  propagated  instantaneously,  it  would 
be  reflected  back  exactly  to  the  place  from  which  it 
started ;  but,  as  it  takes  some  time  for  it  to  pass  twice 
between  the  mirrors,  the  revolving  one  has  slightly 
changed  position,  and  the  reflected  image  will  fall  a 
certain  distance  from  the  slit.  This  small  displacement 
is  accurately  measured,  and  from  it  can  be  obtained 
the  time  that  the  light  requires  to  move  from  one 
mirror  to  the  other.  Great  care,  and  numerous  de- 
vices too  intricate  to  explain  here,  were  used  to  make 
the  result  as  accurate  as  possible.  The  figures  which 
it  is  believed  most  nearly  represent  the  actual  velocity 
of  light  are  299,940  kilometres,  or  186,380  miles,  per 
second. 

The  distance  to  the  sun  obtained  from  this  is  186,380 
X  498  =  92,817,240  miles. 

240.  Telescopes. — -There  are  two  necessary  parts,  of 
every  telescope, — a  mirror,  or  lens,  to  collect  the  light 
and  form  an  image  of  the  object,  and  one  or  more  lenses 
to  magnify  this  image.     When  the  first  of  these  parts 
is  a  mirror,  it  constitutes  a  reflecting  telescope;  when  a 
lens,  a  refracting  telescope.     The  second  part  is  called 
the  eye-piece,  because  the  eye  is  applied  to  it.     The  two 
parts  are  usually  connected  by  a  tube,  to  keep  out 
side-rays. 

241.  Principle  of  Reflectors. — The  essential  part  of  a 
reflecting  telescope  is  a  concave  mirror,  which  collects 
rays  from  all  parts  of  the  object  and  brings  them  to  a 
focus,  forming  an  image  of  the  object.     The  eye  looks 
at  this  image.     As  many  more  rays  of  a  star  can  fall 
on  a  large  mirror  than  on  the  eye,  a  faint  star  will  look 
just  as  much  brighter  as  the  surface  of  the  mirror  ex- 


ASTRONOMICAL  INSTRUMENTS.  283 

ceeds  the  surface  of  the  pupil  of  the  eye,  leaving  out 
some  light  lost  by  the  reflection.  The  largest  mirror  of 
this  kind  ever  made  is  that  of  Lord  Rosse's  telescope  : 
its  diameter  is  six  feet,  and  it  can  collect  250,000  times 
as  much  light  as  the  unaided  eye.  Its  speculum,  as  the 
concave  mirror  is  called,  was  made  of  a  combination  of 
copper  and  tin,  which  was  moulded  and  then  ground 
under  water  till  it  came  exactly  to  the  proper  shape. 
Metallic  specula  of  this  kind  tarnish  soon,  and  then 
have  to  be  taken  from  the  tube  and  reground,  so  that 
few  of  them  are  now  made.  Instead  of  this,  one  side 
of  a  large  glass  disk  is  carefully  hollowed  out  to  the 
proper  shape  and  covered  with  a  very  thin  coating  of 
silver.  This  does  not  soon  tarnish,  and  when  it  does 
the  silver  is  easily  removed  and  a  new  coating  applied. 
Owing  to  the  difficulty  of  supporting  a  piece  of  glass 
of  very  large  size,  reflectors  of  this  kind  have  not  been 
made  over  three  feet  in  diameter. 

242.  Kinds  of  Reflectors. — There  are  three  kinds  of 
reflecting  telescopes,  depending  on  the  situation  of  the 
eye-piece.  The  first  was  invented  by  James  Gregory,1 
the  second  by  Sir  Isaac  Newton,  and  the  third  by  Sir 
William  Herschel ;  hence  their  names. 

In  the  Gregorian  the  rays,  after  being  reflected  by  the 
large  mirror,  are  collected  on  a  smaller  one,  situated 
in  the  position  of  mn,  Fig.  77,  but  so  placed  as  to  re- 
flect the  rays  directly  back  to  M.  The  eye  is  placed 
back  of  the  speculum  and  looks  through  an  opening 
in  it. 

In  the  Newtonian  the  second  mirror  is  placed  diago- 
nally, so  that  the  rays  are  reflected  out  at  one  side  of  the 

1  A  Scotch  mathematician,  1638-1675. 


284 


ASTRONOMY. 


Fio.  76.— NEWTONIAN  REFLECTOR  EQUATOEIALLY  MOUNTED. 


ASTRONOMICAL  INSTRUMENTS.  285 

tube  where  the  eye-piece  is  placed.  The  observer  looks 
at  right  angles  to  the  direction  of  the  object  which  he 
wishes  to  view.  Fig.  77  shows  the  course  of  the  rays 
of  light  through  a  Newtonian  telescope.  M  is  the  con- 
cave speculum,  and  mn  the  diagonal  mirror,  or  "  flat," 
which  reflects  to  the  eye  at  D. 

In  the  Herschelian  the  large  mirror  is  tilted  so  as  to 
bring  the  light  to  a  focus  at  one  edge  of  the  opposite 
end  of  the  tube.  The  observer  is  situated  here,  and 
has  his  back  turned  towards  the  object  he  is  viewing. 


FIG.  77. — PRINCIPLE  OP  THE  NEWTONIAN  REFLECTOR. 

In  the  first  two  the  small  mirror  cuts  off  a  portion 
of  the  light  which  would  otherwise  fall  on  the  specu- 
lum ;  some  light  is  also  lost  by  the  double  reflection. 
In  the  third  the  observer's  head  cuts  off  some  light, 
— less,  however,  than  is  lost  in  the  others.  The  large 
telescope  of  Lord  Rosse  is  Newtonian,  as  are  also 
most  of  those  now  constructed. 

243.  Principle  of  Refractors. — In  refracting  telescopes 
the  light  is  collected  by  means  of  a  double  convex  lens 
of  glass.  The  observer  looks  directly  towards  the  ob- 
ject to  be  viewed,  as  in  the  common  spy-glass.  The 
large  lens  is  called  an  objective,  or  object-glass. 

When  the  early  telescopes  were  made,  a  difficulty  was 


286  ASTRONOMY. 


experienced  from  the  fact  that  the  object-glass  not  only 
refracted  the  rays  and  brought  them  to  a  focus,  but  also 
dispersed  them,  so  that  the  observer  saw  colors  sur- 
rounding the  object  viewed.  This  was  corrected  by 
Dollond1  in  the  following  manner.  He  made  a  double 
convex  lens  of  crown  glass  in  the  usual  way,  and  com- 
bined with  it  a  concave  lens  of  flint  glass.  The  flint 
glass  unites  again  the  different-colored  rays  separated 
by  the  crown  glass,  while  from  its  different  quality  it 
does  not  wholly  counteract  the  refracting  tendency  of 
the  convex  lens.  The  noted  opticians,  Alvan  Clark  & 
Sons,  of  Cambridge,  Massachusetts,  make  their  lenses 
now  as  shown  at  A,  Fig.  79,  combining  a  double  con- 
vex lens  of  crown  with  a  lens  of  flint,  of  such  a  cur- 
vature on  one  side  as  to  fit  into  the  convexity  of  the 
crown,  and  flat  on  the  opposite  side.  The  best  tele- 
scopic work  is  now  done  by  refracting  telescopes  with 
their  objectives  arranged  in  this  way. 

244.  Eye-Pieces. — The  eye-piece  in  a  microscope  for 
magnifying  the  image  formed  by  the  speculum  or  ob- 
jective. One  lens  would  answer  the  purpose,  but,  to 
secure  distinctness  all  around  the  field  of  view,  a  second 
lens  is  added.  The  amount  of  convexity  of  these  lenses 
determines  the  magnifying  power  of  the  telescope.  If 
nearly  flat,  the  image  is  seen  almost  of  its  real  size ;  if 
more  convex,  the  rays  enter  the  eye  so  as  to  make  a 
larger  angle  with  one  another,  and  the  image  is  much 
magnified. 

Fig.  79  gives  the  course  of  rays  through  a  refracting 
telescope.  It  will  be  seen  that  they  cross  at  the  focus 

1  An  English  optician,  1706-1761. 


ASTRONOMICAL  INSTRUMENTS. 


287 


B :  hence  such  a  telescope  always  inverts  objects.  This 
is  a  matter  of  no  consequence  with  the  heavenly  bodies, 


FIG.  78. — PORTABLE  NEWTONIAN  REFLECTOR. 


but  when  terrestrial  objects  are  to  be  observed  it  is 
necessary  to  add  two  more  lenses  to  turn  them  over 


288  ASTRONOMY. 


again.     This  is  the  only  difference  between  an  astro- 
nomical telescope  and  a  common  spy-glass. 


FIG.  79.— ILLUSTRATION  OF  THE  PRINCIPLE  OP  REFRACTORS. 

245.  Micrometer. — Very  fine  spider-webs  are  stretched 
across  the  tube  in  the  focus.     These  can  be  seen  at  the 
same  time  with  the  image  of  the  body  we  are  observing. 
By  having  these  movable,  they  can  be  so  placed  as  to 
agree  with  the  images  of  two  stars  which  may  be  in 
the  field  of  view,  and  the  distance  between  them  may 
be  measured  on  some  scale  conveniently  arranged  for 
the  purpose.    Such  an  instrument  is  called  a  micrometer, 
and  is  indispensable  in  measuring  double  stars  and  for 
other  purposes.     It  is  so  arranged  that  it  can  be  taken 
off  the  telescope  and  put  on  at  pleasure. 

246.  Illuminating  Power. — The   advantages   of  tele- 
scopes are  twofold:  they  collect  a  great  amount  of 
light,  and  they  enable  us  to  see  a  magnified  image  of 
the  object.     The  first  advantage  will  depend  entirely 
on  the  size  of  the  mirror  or  lens.     This  may  be  con- 
sidered to  be  a  huge  eye,  and  all  the  light  which  falls 
on  it  is  conveyed  through  the  eye-piece  to  the  retina. 
Hence  a  great  advantage  of  a  large  telescope  is  the 
ability  to  see  very  faint  objects.     Herschel's  great  re- 
flectors, which  he  made  with  his  own  hands,  brought 
to  his  view  thousands  of  nebulae  which  were  not  pre- 
viously known  to  exist.      The  little  moons  of  Mars 

were  never  recognized  till  Prof.  Hall  saw  them  with 
the  great  refractor  of  the  Washington  Observatory. 


ASTRONOMICAL  INSTRUMENTS.  289 

247.  Magnifying  Power. — The  focal  length  of  the  ob- 
ject-glass, divided  by  the  focal  length  of  the  eye-piece, 
expresses  the  magnifying  power  of  a  telescope.  The 
focal  length  of  a  lens  is  the  distance  from  its  centre  to 
the  place  where  the  image  is  formed.  In  Fig.  79,  AB 
represents  the  focal  length  of  the  lens  A.  We  can 
therefore  increase  the  magnifying  power  either  by 
lengthening  this  distance,  or  by  shortening  the  focal 
length  of  the  eye-piece,  which  is  done  by  making  it 
more  convex.  In  early  times  of  telescope-making  the 
first  method  was  adopted,  and  the  instruments  of  the 
seventeenth  century  were  wonderfully  long  and  un- 
wieldy. Latterly  it  has  been  deemed  better  to  make 
the  telescope  moderately  long,  and  to  gain  power  by 
shortening  the  eye-piece.  If  the  focal  length  of  the 
object-glass  were  forty  inches,  and  that  of  the  eye-piece 
one-half  inch,  the  magnifying  power  would  be  40  -j-  J, 
or  80.  Another  way  to  find  the  magnifying  power  is 
the  following.  Point  the  telescope  to  the  bright  sky 
and  focus  the  eye-piece,  when  a  small  circle  of  light  is 
observable  in  the  eye-piece.  This  is  merely  the  light 
which  falls  on  the  object-glass  reduced  in  size  by  the 
passage  through  the  lenses.  The  diameter  of  this  circle 
divided  into  the  diameter  of  the  object-gla.ss  will  give 
the  magnifying  power. 

It  must  be  remembered  that  the  magnifying  power 
of  a  given  eye-piece  will  vary  with  the  object-glass  with 
which  it  is  connected ;  also  that  there  is  a  limit  to  the 
power  that  can  be  used  with  any  size  of  aperture.  If 
too  great  a  power  is  'applied,  the  magnified  image  be- 
comes indistinct.  It  is  like  looking  through  a  pin- 
hole:  everything  is  confused.  A  refractor  of  six  inches 
aperture  cannot  to  advantage  have  a  power  of  over 

20 


290  ASTEONOMY. 


600 ;  one  of  ten  inches  aperture,  of  over  1000 ;  and  so 
on.  And  this  high  power  can  be  used  only  when  the 
atmosphere  is  in  a  very  favorable  condition. 

In  looking  over  the  country  on  a  hot  day  there  may 
be  noticed  a  quivering  of  the  objects  in  the  horizon. 
This  is  due  to  the  light  from  these  objects  passing 
through  strata  of  air  which  are  differently  heated. 
This  quivering  is  often  noticed  in  the  telescope,  and 
the  higher  the  magnifying  power  the  more  it  interferes 
with  distinct  vision.  The  nights  are  very  few  when 
the  atmosphere  is  so  steady  that  a  very  high  power 
can  be  used  to  advantage.  Sometimes  the  air  in  and 
around  the  telescope-tube  becomes  heated  so  that  noth- 
ing can  be  done  till  the  observatory  is  completely  cooled 
to  the  surrounding  temperature.  The  temperature  in 
the  telescope-room  should  always  be  as  nearly  as  pos- 
sible the  same  as  that  outside. 

248.  Equatorial  Telescopes. —  Small  telescopes  which 
require  to  be  moved  from  one  place  to  another  are 
mounted  on  a  tripod  or  other  light  stand.  In  obser- 
vatories it  is  necessary  to  have  them  permanent.  All 
telescopes  intended  for  general  work  are  mounted 
equatorially.  An  equatorial  telescope  is  shown  in  Fig. 
80.  The  advantage  of  this  mounting  is  that  a  star  can 
be  easily  followed  as  it  is  carried  by  the  diurnal  motion 
around  the  earth.  The  mounting  consists  of  two  axes 
at  right  angles  to  each  other :  one  of  these  is  pointed 
directly  towards  the  pole  of  the  heavens,  and  is  called 
the  polar  axis  ;  the  other  is  attached  to  this  at  one  end, 
and  is  called  the  declination  axis. '  The  telescope  turns 
on  the  declination  axis,  and  with  it  around  the  polar 
axis :  hence  it  can  be  pointed  to  any  part  of  the  sky. 
As  the  polar  axis  is  parallel  to  the  axis  about  which 


ASTRONOMICAL  INSTRUMENTS. 


291 


the  diurnal  motion  of  the  stars  is  performed,  the  tele- 
scope pointed  to  a  star  and  turned  on  this  axis  alone 


FIG.  80. — EQUATORIAL  KEFRACTINQ  TELESCOPE. 

will  follow  the  star  from  rising  to  setting.     This  turn- 
ing can  be  done  by  clock-work  so  regulated  as  to  move 


292 


ASTRONOMY. 


FIG.  81.— LICK  OBSERVATORY,  MOUNT  HAMILTON,  CALIFORNIA. 
(Glass  mad   by  Clark,  of  Cambridge,  Mass. ;  mounted  by  Warner  wiri  Swazey,  Cleveland,  0.) 


ASTRONOMICAL  INSTRUMENTS.  293 

the  telescope  just  as  fast  as  the  star  moves ;  that  is, 
at  such  a  rate  as  to  make  a  complete  revolution  in  a 
day.  The  star  then  will  keep  exactly  in  the  field  of 
view,  and,  if  the  clock  works  accurately,  on  the  same 
spider-line  of  the  micrometer,  so  that  it  can  be  studied 
and  measured  at  leisure.  In  the  figure,  AB  is  the  polar 
axis,  CD  is  the  declination  axis,  E  is  the  finder,1  F  is  a 
lamp  so  arranged  as  to  light  up  the  interior  of  the 
tube  so  that  the  spider-lines  can  be  seen  at  night,  G 
and  G'  are  graduated  circles  upon  which  the  distance 
of  a  star  from  the  meridian  and  its  declination  may  be 
read,  H  is  the  clock  which  turns  the  telescope,  J  is  the 
eye-piece  and  micrometer. 

249.  Transit  Instrument. — The  transit  instrument  is  a 
telescope  mounted  on  a  single  axis  which  rests  on  two 
piers  or  posts.  It  is  set  so  as  to  swing  exactly  in  the 
meridian;  hence  the  axis  must  point  east-and-west. 
In  the  eye-piece  of  the  telescope  is  a  series  of  parallel 
spider-lines,  which  are  stretched  across  vertically,  and 
one  or  two  horizontal  lines.  The  pivots  on  which  the 
axis  rests  are  ground  very  carefully,  so  as  to  be  exactly 
circular  and  of  equal  size. 

The  use  of  the  transit  instrument  is  to  record  the 
passage  of  stars  over  the  meridian,  and  thus  find  the 
true  sidereal  time.  It  will  be  remembered  that  when 
the  vernal  equinox  crosses  the  meridian  the  sidereal 
clock  indicates  0  h.  0  m.  0  s. ;  also  that  a  star  situated 
at  this  point  would  have  its  right  ascension  0  h.  0  m. 

1  A  finder  is  a  little  telescope  with  a  large  field  of  view,  by  which 
to  find  a  star.  As  it  will  embrace  a  large  circle  of  the  heavens,  the 
star  can  be  easily  found  and  placed  in  the  centre  of  the  field  of  view. 
It  can  then  be  seen  in  the  large  telescope.  A  finder  is  indispensable 
to  any  telescope  except  the  smallest. 

25* 


294 


ASTRONOMY. 


Os.  If  a  star  pass  the  meridian,  for  instance,  1  h, 
27  m.  after  this,  its  right  ascension  is  1  h.  27  m.  If 
this  star  be  observed  at  its  passage  over  the  meridian, 


FIG.  82.— PORTABLE  TRANSIT  INSTRUMENT. 


and  the  time  recorded  by  an  accurate  sidereal  clock, 
this  clock  will  indicate  1  h.  27  m.  If  it  do  not,  it  is 
in  error,  and  the  amount  of  its  error  becomes  known. 


ASTRONOMICAL  INSTRUMENTS.  295 

We  thus  have  the  opportunity  of  correcting  our  side- 
real clocks  if  we  know  the  exact  right  ascensions  of 
certain  stars.  These  right  ascensions  are  given  in  the 
Nautical  Almanac.  The  observer  fixes  the  telescope 
to  the  point  where  the  star  will  cross,  and  notes  the 
time  of  passage  over  each  of  the  vertical  spider-lines. 
The  average  of  all  these  times  will  be  the  time  when 
it  crosses  the  meridian,  if  the  instrument  be  accurately 
adjusted;  if  not,  certain  corrections  must  be  applied. 
This  gives  him  the  clock  time  of  passage.  He  then 
compares  this  with  the  right  ascension  of  the  star  as 
given  in  the  Nautical  Almanac;  the  difference  is  the 
error  of  the  clock.  From  the  sidereal  time  the  mean 
solar  time  can  be  calculated.  A  small  transit  instru- 
ment is  shown  in  Fig.  82. 

The  Camera. — This  must  now  be  considered  an 
astronomical  instrument  of  value.  Its  advantages 
over  the  human  eye  are, — 

1st.  It  does  not  get  tired.  By  leaving  the  plate  ex- 
posed a  long  time,  the  telescope  being  kept  pointed  at 
the  object  by  a  driving  clock,  the  impressions  are 
strengthened,  so  that  finally  objects  below  the  ken  of 
direct  vision  may  be  observed. 

2d.  It  takes  a  quicker  look.  An  object  in  rapid 
motion — a  cannon-ball  or  flash  of  lightning — can  be 
photographed. 

3d.  It  gives  a  perfectly  accurate  map  of  the  relative 
positions  of  objects,  and  saves  the  labor  of  making 
drawings. 

Perfect  steadiness  and  uniformity  of  motion  on  the 
part  of  the  telescope  are  required,  especially  for  long 
exposures.  To  see  that  the  object  keeps  exactly  in  the 
field  of  view,  it  is  customary  to  have  telescopes  which 


296 


ASTRONOMY. 


are  arranged  for  photographing  made  with  two  tubes 
side  by  side,  attached  together.  While  one  is  photo- 
graphing the  object,  the  observer  directs  the  motion 
through  the  other. 

250.  Sextant. — The  sextant  is  an  instrument  for 
taking  angles  on  shipboard,  or  in  other  places  where  a 
fixed  telescope  cannot  be  arranged.  It  is  shown  in  Fig. 
83.  It  consists  of  a  graduated  scale,  AA,  usually  about 
60°  in  length,  but  divided  into  one  hundred  and  twenty 
parts.  Another  scale  works  on  this,  which  is  attached 
to  the  arm  B;  on  this  arm  is  a  mirror,  C;  another 
piece  of  glass,  D,  of  which  one-half  is  silvered  and  the 
other  half  clear,  is  fixed  to  the  frame  of  the  instrument. 


FIG.  83.— SEXTANT. 


The  telescope  E  points  directly  to  this  second  piece  01 
glass.  If  now  it  be  desired  to  read  an  angle, — for  in- 
stance, to  know  the  height  of  a  given  star  above  the 
horizon  at  sea, — the  sextant  is  held  by  the  handle,  so 
that  the  observer,  looking  through  the  clear  part  of  the 
glass  D,  sees  the  horizon.  Then  he  moves  the  index 


ASTRONOMICAL  INSTRUMENTS.  297 

on  the  pivot  till  the  star  reflected  from  the  mirror  C 
and  again  from  the  silvered  part  of  D  agrees  with  the 
horizon.  The  angle  is  then  read  on  the  scale. 

Should  the  observer  be  on  land,  the  horizon  is  so 
broken  that  no  definite  point  can  be  taken.  Then  it  is 
necessary  to  read  the  angle  between  the  star  and  its 
reflection  from  a  vessel  of  mercury.  This  gives  double 
the  altitude  of  the  star.1 

The  Astronomical  Clock,  and  the  Chronometer. — This  is 
simply  an  unusually  good  common  clock  which  beats 
seconds.  The  dial-plate  reads  up  to  24  instead  of  12, 
and  the  hour-hand  makes  one  revolution  instead  of 
two  in  twenty-four  hours.  It  keeps  sidereal  rather 
than  mean  solar  time.  Clock  time  may  be  obtained 
from  it  by  computation. 

The  error  of  a  clock  is  the  amount  it  differs  from  the 
true  time.  Its  rate  is  its  gain  or  loss  per  day.  A  good 
clock  must  have  a  uniform  rate. 

The  clock  is  often  placed  so  that  the  observer  at  the 
transit  instrument  can  hear  it  beat  seconds.  With  his 
eye  on  the  star,  his  ear  can  follow  the  beats  of  the 
clock,  and  the  time  of  a  star's  crossing  a  wire  can  be 
determined,  by  a  skilled  observer,  to  one-tenth  of  a 
second.  It  is  sometimes  more  convenient  to  record 
the  transits  by  electric  connection  on  an  instrument 
called  a  chronograph,  from  which  the  times  can  be  read 
at  leisure. 

A  chronometer  is  a  large  watch  made  with  extreme 
care,  and  set  in  rings,  so  that  it  will  preserve  a  horizon- 
tal position  on  a  rolling  vessel.  It  is  used  to  determine 
longitude,  as  already  explained,  and,  after  being  rated 
at  an  observatory,  is  carried  by  the  ship  on  its  voyage. 

1  Why  is  this  ? 


298  ASTRONOMY. 

251.  Spectrum  Analysis. — The  phenomena  connected 
with  the  dispersion  of  light  have  performed  a  very 
important  part  in  modern  astronomical  research.  By 
them  we  have  been  enabled  to  tell  the  construction  of 
our  sun,  and  of  all  the  other  suns  which  crowd  our  skies; 
we  have  been  able  to  say  what  elements  compose  them, 
and  in  what  form  those  elements  exist;  we  have  found 
out  that  the  nebulae  are  different  in  constitution  from 
the  stars ;  that  planets  shine  by  reflected  light,  and,  to 
some  extent,  the  character  of  their  atmospheres ;  that 
some  stars  are  moving  towards  us  or  away  from  us, 
and,  approximately,  the  velocity  of  their  motion.  All 
this  information  has  been  contained  in  the  rays  of  light 
which  have  fallen  on  the  earth  since  its  creation,  but 
only  within  about  a  quarter  of  a  century  have  we  been 
able  to  understand  it. 

We  will  now  explain  briefly  the  principles  on  which 
the  science  of  spectrum  analysis  is  founded. 

It  has  been  known  since  the  time  of  Sir  Isaac  New- 
ton that  light,  when  passed  through  a  prism,  is  divided 
into  its  several  parts.  The  violet  rays  are  turned  aside 
the  most,  and  the  red  least.  This  phenomenon  is  seen 
whether  the  light  comes  to  the  prism  from  the  sun  or 
from  a  burning  candle  or  from  the  electric  light.  If 
this  light  falls  on  the  prism  after  passing  through  a 
narrow  slit,  it  is  spread  out  into  a  band  of  colors,  which 
is  called  a  spectrum.  This  is  nothing  more  than  a  slice 
of  a  rainbow  cut  crosswise.  Now,  so  long  as  the  source 
is  a  glowing  solid  or  liquid,  or  a  very  greatly  condensed 
gas,  we  obtain  just  such  a  spectrum.  A  heated  piece 
of  lime  will  give  us  exactly  the  same  spectrum  as  glow- 
ing carbon,  such  as  we  have  in  a  candle-flame.  But 
suppose  we  pass  the  light  from  a  glowing  gas  in  ordi- 


ASTRONOMICAL  INSTRUMENTS.  299 

nary  state  through  a  slit  and  a  prism.  The  spectrum 
now  changes.  Instead  of  a  combination  of  colors  run- 
ning into  one  another  we  have  narrow  bright  bands  or 
lines  of  color,  which  are  separated  by  dark  intervals. 
Moreover,  each  gas  has  its  own  peculiar  set  of  bright 
lines.  Thus,  sodium  has  only  two  yellow  lines,  close 
together,  while  the  spectrum  of  iron  is  composed  of 
hundreds  of  lines  of  all  colors.  If,  then,  we  desire  to 
know  the  elements  of  which  any  substance  is  composed, 
we  may  apply  enough  heat  to  vaporize  these  elements, 
allow  the  light  to  pass  through  a  slit  and  a  prism,  and 
see  what  is  the  position  of  the  bright  lines  formed. 
These  must  then  be  compared  with  known  spectra. 
If  we  get  the  two  sodium  lines,  for  instance,  we  know 
sodium  to  exist  in  the  substance  examined. 

There  is  one  other  case  to  be  considered, — that  in 
which  light  from  a  solid  or  a  liquid  passes  through  a 
gas  before  it  reaches  the  prism.  Here  the  gas  absorbs 
some  of  the  rays  of  the  light,  and  it  is  found  that  it 
absorbs  exactly  the  same  rays  that  it  gives  out  when  it 
is  itself  heated  to  glowing.  The  spectrum  formed  is 
then  a  continuous  spectrum,  similar  to  that  given  out 
by  a  solid  or  a  liquid,  but  crossed  by  a  series  of  narrow 
dark  lines,  and  these  lines  have  exactly  the  same  posi- 
tion as  the  .bright  lines  which  the  gas  forms  when  self- 
luminous.  Let  us  suppose  that  light  from  a  candle 
or  from  white-hot  iron  passes  through  sodium  vapor. 
After  emerging  from  the  prism  the  spectrum  would  be 
the  ordinary  spectrum,  except  that  in  the  yellow  por- 
tion there  would  be  two  dark  lines  agreeing  exactly  in 
position  and  relative  character  with  the  bright  lines 
previously  mentioned.  If,  then,  we  have  a  spectrum 
which  is  crossed  by  dark  lines,  we  know  that  the  light 


500  ASTRONOMY. 


comes  from  a  solid  or  a  liquid,  or  a  very  dense  gas,  and 
passes  through  a  less  bright  atmosphere  of  gas. 

252.  Fundamental  Principles. — The  principles  of  spec- 
trum analysis  thus  deduced  are, — 

1.  A  glowing  solid,  liquid,  or  compressed  gaseous 
body  gives  a  continuous  spectrum. 

2.  A  glowing  gas  under  low  pressure  gives  a  spec- 
trum of  bright  lines   only,  each  element  having  its 
peculiar  lines. 

3.  Light  which  comes  from  a  glowing  solid  or  liquid, 
or  compressed  gas,  and  passes  through  less  bright  gas, 
gives  a  spectrum  crossed  by  dark  lines,  and  these  dark 
lines  agree  exactly  in  position  and  character  with  the 
bright  lines  given  out  by  the  same  gas.1 

253.  Application  to  the  Heavenly  Bodies. — These  prin- 
ciples being  established  by  experiments  with  terrestrial 
substances,  we  have  only  to  examine  the  spectra  ob- 
tained from  the  light  from  the  heavenly  bodies  to  tell 
what  is  their  constitution  and  composition. 

The  solar  spectrum  is  continuous,  but  crossed  by 
dark  lines.  Hence  we  infer  that  the  photosphere  of  the 
sun  is  solid  or  liquid,  or  a  gas  condensed  by  the  enor- 
mous pressure  upon  it,  and  is  surrounded  by  an  atmos- 
phere through  which  the  rays  that  reach  us  pass.  The 

1  As  the  gas  which  is  the  source  of  light  becomes  more  dense  and 
approaches  a  liquid  in  character,  the  lines  of  its  spectrum  broaden 
into  bands,  and  when  condensation  is  complete  the  bands  run  into  one 
another  and  so  form  a  continuous  spectrum.  It  will  thus  be  seen 
that  there  is  no  distinct  line  between  the  different  kinds  of  spectra. 

It  is  supposed  that  the  change  from  one  kind  of  spectrum  to 
another  is  due  to  a  change  in  the  complexity  of  the  molecules  of  the 
substance  examined.  A  simple  substance  gives  a  spectrum  of  lines. 
When  the  complexity  increases  by  cooling  down,  the  spectrum  changes 
first  to  a  band  and  then  to  a  continuous  spectrum. 


ASTRONOMICAL  INSTRUMENTS.  301 

dark  lines  which  can  be  seen  in  the  spectrum  agree 
with  the  bright  lines  of  hydrogen,  sodium,  magnesium, 
and  other  substances  on  the  earth.  Hence  we  infer 
that  these  elements  exist  in  the  chromosphere  of  the 
sun ;  that  is,  that  'great  quantities  of  sodium,  etc.,  are 
burning  less  brightly  than  the  sun,  and  the  sunlight 
passes  through  these  vapors  before  it  reaches  the  earth. 

The  spectra  of  the  stars  are  fainter  than  those  of  the 
sun,  but  are  of  the  same  general  character.  They  are 
crossed  by  dark  lines  in  the  same  way ;  the  substances 
are  not  identical,  and  there  is  a  .slight  diversity  in  the 
composition  of  different  ones ;  but  all  that  have  been 
examined  show  many  terrestrial  elements,  thus  proving 
that  all  through  the  universe  there  is  the  same  kind  of 
material.  In  general  the  red  stars  present  a  different 
kind  of  spectrum  from  the  yellow,  and  the  yellow  from 
the  white.  Those  stars  which  are  of  the  same  color 
have  the  same  kind  of  spectra.  It  is  considered  that 
these  differences  indicate  different  stages  of  star-life. 
Thus,  Capella  and  our  sun  are  of  the  same  color,  and 
have  almost  exactly  the  same  character  of  spectra.  We 
thence  infer  that  they  are  in  the  same  condition. 

The  spectra  of  the  nebulae  are  some  of  them  con- 
tinuous spectra.  Such  nebulae  are  then  probably  solid 
bodies,  collections  of  suns.  Others  show  spectra  of 
bright  lines  only.  These  are  then,  according  to  our 
second  principle,  masses  of  glowing  gas,  which  has 
not  yet  condensed  into  anything  like  suns.  The  Great 
Nebula  of  Andromeda  belongs  to  the  former  class; 
that  of  Orion  to  the  latter. 

The  planets,  since  they  are  visible  by  reflected  sun- 
light, give  the  same  spectra  as  the  sun,  with  the  addi- 
tion to  it  of  some  dark  lines,  which  are  made  by  ab- 

26 


302 


ASTRONOMY. 


sorption  of  the  planet's  atmosphere.  The  spectrum  of 
the  moon  shows  none  of  these  dark  lines,  but  is  simply 
a  fainter  solar  spectrum,  because  the  moon  probably 
has  no  atmosphere,  and  does  not  give  out  or  aftect 
in  any  way  the  sunlight  which  it  reflects.  All  the 
comets  examined  show  a  spectrum  of  bright  lines  on 
top  of  a  continuous  one,  indicating  a  solid  nucleus  and 
a  large  amount  of  self-luminous  gas  surrounding  it. 
This  gas  seems  to  be  a  hydrocarbon. 


Pro.  84.— PORTION  OF  A  SPECTROSCOPE. 

254.  Spectroscopes. — Fig.  84  shows  the  theory  of  the 
spectroscope.  The  light  from  the  source  comes  in 
through  one  of  the  tubes  after  passing  through  a  slit. 
It  then  passes  through  one  or  more  prisms, — six  in  the 
figure, — each  one  farther  separating  the  rays  from  one 
another.  A  telescope  then  magnifies  the  spectrum 
formed  by  the  prisms.  When  we  are  examining  a  star, 
this  whole  apparatus  is  attached  to  the  eye-end  of  a 


ASTRONOMICAL  INSTRUMENTS.  303 

telescope  by  the  tube  B,  so  that  the  slit  shall  be  in  the 
focus  of  the  object-glass.  The  observer  looks  in  at  A, 
and  sees  the  light  of  the  star,  which  has  passed  around 
through  all  the  prisms,  separated  into  its  primary  colors, 
with  its  dark  lines  in  their  appropriate  positions.  The 
more  prisms  there  are  in  the  train,  the  more  the  light 
will  be  dispersed  and  the  longer  will  be  the  spectrum, 
but,  being  spread  over  more  surface,  it  will  be  fainter, 


APPENDICES. 


I. 

LIST  OF  LARGE  TELESCOPES. 

L— REFRACTORS    OF    OVER    FIFTEEN    INCHES' 
APEETURE. 


Name  and  Place. 

a 

II 

•< 

Maker. 

Lick  Observatory  California 

36 

Clark  of  Cambridge  Mass 

Imperial  Observatory  Russia  

30 

30 

Yale  College  Observatorv  

28 

Clark. 

27 

Grubb  of  Dublin 

Naval  Observatory,  Washington  

26 

Clark. 

McCormick  Observatory,  University  of  Va... 
Gatesh  cad  Observatory   England  

26 

25 

T.  Cooke,  of  England. 

Paris  Observatory  

236 

Princeton  College  Observatory  

23 

Clark. 

Denver  Observatory  

20 

20 

Grubb 

19 

Chicago  Observatory  

18.5 

Clark 

Private  Observatory  Buffalo  N  Y 

18 

Fitz  of  New  York 

Warner  Observatory,  Rochester  

16 

Clark. 

Carleton  College  Observatory,  Minn  
Washburne  Observatory,  Madison,  Wis  

16 
15.5 
15 

Brashear. 
Clark. 
Merz  &  Mahler 

15 

15 

Grubb 

15 

<c 

26* 


305 


306 


APPENDICES. 


II.— REFLECTORS  OF   OVER  TWO   FEET   APERTURE. 


Name  and  Place. 

Aperture,  in 
Inches. 

Maker. 

Earl  of  Rosse,  Parsonstown,  Ireland  

72 

Earl  of  rfosse. 

Melbourne  Observatory  Australia 

48 

Grubb 

47 

Martin  A  Eichens. 

Mr.  Common's  Observatory,  England  
Marseilles  Observatory  France                 .... 

36 
31  5 

31  5 

c« 

Henry  Draper's  Private  Observatory,  Has- 
tings, N.Y  

28 

H   Draper 

Harvard  College          

28 

28 

Mr.  Lassell,  Maidenhead,  England  
Cambridge,  England  

24 
24 

Mr.  Lassell. 

, 

n. 
ASTRONOMICAL  SYMBOLS. 

The  following  are  the  symbols  and  abbreviations  used  in  ordi- 
nary almanacs. 

SIGNS  OF   THE  PLANETS,  ETC. 


©  The  Sun. 
<I  The  Moon. 
0  Mercury. 
9  Venus, 
or  6   The  Earth. 


cf  Mars. 
%  Jupiter. 
h  Saturn. 

5  Uranus. 
W  Neptune: 


THE   MOON'S  PHASES. 

New  Moon  (Conjunction).         Q  Full  ^oon  (Opposition). 
First  Quarter.  C    Last  Quarter. 

SIGNS  OF  THE  ECLIPTIC. 


Spring 
signs. 

Summer 
signs. 


6. 


cy>  Aries. 
ft  Taurus, 
n  Gemini. 
05  Cancer. 
$1  Leo. 
ttjj  Virgo. 


Autumn 
signs. 

Winter 


7.  =s=  Libra. 

8.  TT\,  Scorpio. 

9.  /  Sagittarius. 
10.  V?  Capricornus. 

Aquarius. 
12.  X  Pisces. 


flO 

11 

1 12 


APPENDICES. 


307 


ABBKEVIATIONS. 


Q  Ascending  Node. 

13  Descending  Node. 

6  Conjunction. 

O  Quadrature. 

8  Opposition. 

E.  A;  Eight  Ascension. 

Dec.  Declination. 


Minutes  of  Arc. 
Seconds  of  Arc. 
Hours. 

Minutes  of  Time. 
Seconds  of  Time. 


THE   GEEEK   LETTEES. 

In  astronomy,  used  principally  to  designate  the  different  stars 
in  each  constellation. 


Letter.   Name. 

Letter 

Name. 

a 

Alpha. 

V 

Nu. 

ft 

Beta. 

£ 

Xi. 

y 

Gamma. 

0 

Omi'kron. 

6 

Delta. 

IT 

Pi. 

9 

Epsi'lon. 

P 

Eho. 

c 

Zeta. 

a 

Sigma. 

9 

Eta. 

T 

Tau. 

0 

Theta. 

V 

Upsi'lon. 

I 

lo'ta. 

$ 

Phi. 

K 

Kappa. 

X 

Chi. 

A 

Lambda. 

f 

Psi. 

u 

Mu. 

« 

Ome'ga. 

III. 
LENGTHS  OF  DAYS,  MONTHS,  AND  YEARS. 

24h  =  a  mean  solar  day ;  the  ordinary  day. 

23h  56m  4.09"     =  a  sidereal  day  ;  the  exact  time  of  the  earth's  rotation. 
29.53088  days  =  a  mean  synodical  month ;  the  common  lunar  month, 

being  the  time  from  one  new  moon  until  the  next, 

or  from  one  full  moon  until  the  next. 
27.32166  days  =  a  sidereal  month  ;  the  time  of  the  revolution  of  the 

moon  about  the  earth. 
365.24220  days  =  a  tropical  year ;  the  common  year,  being  the  time 

from  one  vernal  equinox  until  the  next. 
365.25636  days  =  a  sidereal  year ;  the  time  of  the  revolution  of  the 

earth  about  the  sun. 


308 


APPENDICES. 


fj 

•i  *' 

> 


o 
o 


at 

II 


CQ 


H 


i 


W 

EH 


'*  00' 
*O  r^  < 
OS  01  < 


m  up 

i_<  ^  <B  M     s  SB  »r 


mi 

B.S'SH 


O   t-COOr-lr-l<NOfH 


5  O  O  O  O  O  c 


00  CO  (N  •*  l>  CO  i-J  •*  •* 
^•^US^'rH        PH  r-(  r-  CO 


APPENDICES. 


309 


PERIODIC  COMETS. 


Name. 

Last 
Perihelion 
Passage. 

Periodic 
Time. 

Notes. 

Encke 

1891 

3  30  years 

See  page  198 

Tempel  I  

1888 

5.20      " 

1889 

535      " 

Discovered  in  1884 

Swift  
Brorsen  

1891 
1890 

5.50      " 
5.56      " 

Seen  in  1869,  and  not  known  to  be  pe- 
riodic till  1880. 
Discovered  in  1846. 

Winnecke  
Tempel  II  

1891 
1891 

5.64      " 
6.00      " 

Discovered  in  1819,  and  not  seen  again 
till  1858. 

D'Arrest  
Biela 

1890 
f 

6.39      " 
660      " 

Faintest  of  periodic  comets. 
See  page  200 

Wolf. 

1891 

670      " 

Discovered  in  1884 

Faye  

1887 

741      " 

Discovered  in  1843 

Denning  

1881 

900(?)" 

Discovered  in  1881 

Tuttle  

Pons-Brooks  
Halley  . 

1885 

1885 
1835 

13.78      " 

71.34      " 
76  37      " 

Discovered  in  1790,  and  not  seen  again 
till  1858. 
Discovered  in  1885. 
See  page  198 

310 


APPENDICES. 


LIST  OF  NOTED  DOUBLE  STARS. 

COMPILED  FROM    "HAND-BOOK  OF    DOUBLE    STARS' 
OF   GLEDHILL,  CROSSLEY,  AND   WILSON. 


Name. 

R.  A. 

Dec. 

Position 
Angle. 

Bis- 

tance. 

Magnitude. 

Remarks. 

51  Piscium  

h.  m. 
0  26.2 
0  418 

o     / 
6  17 
67  11 

0 

82 
145 

n 

28 
67 

5       9 
4       76 

White,  ash. 
Yellow,  purple. 

Polaris  

1  13.7 

88  40 

213 

186 

2       9 

Binary. 
Yellow,  white. 

1  558 

2  11 

325 

3 

3       4 

White  blue 

y  Andromedai  

t  Trianguli  

i  Cassiopeia}  
v  Geti 

1  56.5 

2    5.4 
2  19.2 
2  37.1 

41  46 

29  44 
66  51 
2  44 

63 

76 
(265 
1108 
291 

10.1 

3.9 
2.11 

»} 

3       5 

5       6.4 
4.2    7.1  8.1 
36    7 

Yellow,  blue.    Bi- 
nary. 
Yellow,  blue. 

Triple.    Binary. 
Yellow,  bine. 

Aldebaran  

4  29 

16  16 

35 

114 

1     11.2 

Bed,  blue. 

Bigel  

5    8.8 
5  29 

—8  20» 
—  5  30 

199 
J 

9.2 
See 

1       8 
I                 J 

Sextuple.  In  neb- 

(r Orionis  

5  32.7 

—2  40 

'"I 

p.  242. 

J...        ...| 

ula  of  Orion. 
Quadruple. 

£  Orionis 

5  347 

—2 

154 

24 

2       57 

Yellow,  red      Bi- 

Stilus 

6  397 

—16  32 

50 

101 

1     10 

nary. 
Small    star,    seen 

M  Canis  Majoris... 
S  Geminorum  
Castor  
38  Lyncis  

6  50.6 
7  13 
7  27 
9  11.4 

—13  53 
22  12 
32    1 
37  19 

342 

204 
234 
240 

3 
7 
6.5 

2.9 

4.7    8 
3.2    8.2 
3       3.5 
4       6.7 

with  difficulty. 
Yellow,  blue. 

Binary.    White. 
White,  blue. 

Y  Leonis  

y  Virginia  

c  Bootis 

10  13.4 

12  35.6 
14  397 

20  27 

—047 
27  35 

112 

160 

329 

3.3 

5 
29 

2       3.5 

3       3 
3       6.3 

Binary.      Yellow, 
red. 
Yellow.    Binary. 
Bed,  lilac. 

£  Bootis  

14  45.8 

19  36 

283 

4.5 

4.7    6.6 

Pale-red,  deep-red. 

fiSerpentis  
Antares  
a  Herculis  
p  Herculis  
TOOphiuchi  

15  29.1 
16  22 
17    9.1 
17  19.5 
17  59.4 

18  404 

10  56 
—26  10 
1432 
37  15 
2  33 

39  33 

186 
273 
115 
312 
78 
f  16 

3.8 
3 
4.7 
3.7 
3.2 
3.01 

3       4 

1.5    7.7 
3       6.1 
4       6.1 
4       6 

White,  ash. 
Bed,  blue. 

White. 
Binary. 

See  page  241 

B  Cveni    .. 

19  258 

27  42 

ll" 

2.5  J 
343 

3       6.3 

Yellow,  blue. 

c  Draconis  
yiDelphini  
61  Cveni 

19  48.6 
20  41.8 
21    14 

69  68 
15  42 
38    7 

360 
272 
117 

3 
11.1 
20 

4       7.6 
4      6 

53    59 

Golden-green. 
Binary. 

21  389 

28  12 

118 

37 

4       6 

White 

f  Aquarii  

22  226 

—0  38 

333 

3.4 

4       4.1 

Minus  sign  means  South  Declination. 


APPENDICES. 


311 


LIST  OF  NOTED  CLUSTERS  AND  NEBULA 

I.— CLUSTERS  SEEN  BY  THE  NAKED  EYE. 

Pleiades  in  Taurus. 

Hyades  in  Taurus. 

Praesepe  in  Cancer. 

Coma  Berenices. 

Clusters  in  Sword-Handle  of  Perseus. 

II.— CLUSTERS  RESOLVABLE   BY  TELESCOPES. 


Constellation. 

B.  A.,  1880. 

Dec.,  1880. 

Bemarks. 

Cassiopeia 

h.   m. 
1    38 

o      / 
60    38 

Seen  by  t\vo-in?)~  telescope 

Auriga  

5    44 

32    32 

6      1 

24    21 

To  naked  eye  faint  nebula 

13      7 

18    48 

Canes  Venatici  

13    37 

28    58 

One  thousand  small  stars 

Libra    ... 

15    12 

2    32 

16    10 

—22    42 

Like  a  comet 

Hercules  

16    37 

36    41 

Sagittarius 

18    29 

24      0 

Antinous  

18    46 

—6    24 

21    24 

11    38 

Bright  cluster 

21    27 

—1    22 

Compressed    and    resolved 

with  difficulty. 

III.— NEBULAE. 


Constellation. 

B.A.,  1880. 

Dec.,  1880. 

Bemarka. 

Great  Nebula  of  Andromeda.. 
Great  Nebula  of  Orion  

Ursa  Major  

h.   m. 
0    36 
5    29 

f  9    39 

o      / 
40    37 
5    29 

12    50) 

See  page  253. 
See  page  253.  Whole  neigh- 
borhood nebulous. 

Virgo  

111      8 
(12    20 

-<  12    24 

55    40] 
13    36) 

8    40  > 

Canes  Venatici  

Il2    34 
13    25 

10    57  j 
27    49 

Spiral  nebula 

Sagittarius  

17    55 

23      2 

Trifid  nebula 

Scutum  Sobieskii  

18    14 

—16    15 

Horseshoe  nebula 

Lyra  

18    49 

32    53 

Ring  nebula    See  page  253 

Dumb-Bell    Nebula   in  Vul- 

19    54 

22    23 

See  page  258 

Delphinns  
Aounrius*..  •••••••••••••••*«••••••••• 

i 

20    28 
20    58 

6    59 
—11    50 

Planetary  nebula. 

INDEX. 


Alexandrian  astronomy,  12. 

Algol,  254. 

Apparent  motions  of  planets,  31. 

Auriga,  231. 

Aurora  borealis,  117. 

Biela's  comet,  200. 

Binary  stars,  250. 

Calendar,  Gregorian,  108. 

Julian,  107. 
Canis  Major,  232. 
Cassiopeia,  231. 
Celestial  equator,  41. 
Celestial  measures,  23. 
Chaldean  astronomy,  10. 
Chinese  astronomy,  10. 
Chromosphere,  48. 
Clusters,  260. 
Colors  of  stars,  228. 
Comets,  189. 

b,  1881,  201. 

Biela's,  200. 

changes  in  head,  197. 

constitution,  196. 

Encke's,  199. 

Halley's,  198. 

orbits,  193. 
Conjunction,  31. 
Constellations,  217. 

description,  230. 

names,  219. 
Copernicus,  15. 
Corona,  47. 
Day,  apparent  solar,  101. 

civil  and  astronomical,  105. 


27 


Day,  mean  solar,  103. 

sidereal,  101. 
Day  and  night,  96. 
Declination,  42. 

Description  of  constellations.  230 
Distribution  of  nebulae,  274. 

of  stars,  272. 
Diurnal  motion,  22. 
Double  stars,  249. 
Earth,  79. 

motions,  85. 

orbit,  87. 

shape,  79. 

size,  81. 

weight  and  density,  83. 
Eclipses,  144. 

moon,  146. 

number,  150. 

sun,  147. 
Ecliptic,  39. 
Ellipse,  33. 
Encke's  comet,  199. 
Equation  of  time,  104. 
Equatorial  telescopes,  290. 
Equinox,  40. 
Eye-pieces,  286. 
First-magnitude  stars,  222. 
Galaxy,  224. 
Galileo,  16. 
Gemini,  233. 

General  view  of  the  heavens,  18. 
Greatest  elongation,  31. 
Greek  astronomy,  11. 
Halley's  comet,  198. 

313 


314 


INDEX. 


Heavens  at  the  equator  and  poles,  24. 

Hercules,  235. 

Hipparchus,  13. 

History  of  astronomy,  9. 

Horizon,  19. 

Illuminating    power    of    telescopes, 

288. 

Inferior  planets,  65. 
Jupiter,  163. 

appearance,  164. 

satellites,  166. 
Kepler,  15. 
Kepler's  Laws,  36. 
Latitude,  109. 
Leo,  234. 
Libration,  130. 
Light,  properties,  279. 

velocity,  280. 
Longitude,  109. 

how  found,  110. 
Lyra,  235. 

Magellanic  clouds,  271. 
Magnifying  power  of  telescopes,  289. 
Magnitudes  of  stars,  220. 
Mars,  153. 

satellites,  156. 
Mercury,  66. 
Meteors,  204. 
Meteor-watching,  210. 
Micrometer,  288. 
Milky  Way,  224. 
Minor  planets,  1 59. 
Mira,  254. 
Month,  106. 
Moon,  124. 

libration,  130. 

orbit,  125. 

phases,  126. 

physical  condition,  132. 
Moons  of  Jupiter,  166. 

Mars,  156. 

Neptune,  185. 

Saturn,  177. 

Uranus,  182. 


Nebulae,  262. 

Nebular  hypothesis,  276. 

Neptune,  183. 

New  stars,  259. 

Newton,  16. 

Nodes  of  planets'  orbits,  43. 

November  and  August  meteors,  207. 

Nutation,  91 

Obliquity  of  ecliptic,  100. 

Occupations,  139. 

Opposition,  31. 

Orbits  of  planets,  32. 

Orion,  232. 

Parabola,  33. 

Parallax  of  stars,  225. 

sun,  44. 
Phenomena    of    Jupiter's    satellites, 

168. 

Photosphere,  50. 
Planetoids,  159. 
Planets,  inhabited,  186. 

orbits  of,  32. 

statistics  of,  35. 
Precession  of  equinoxes,  89. 
Proper  motion  of  stars,  227. 
Ptolemy,  13. 

Radiant  point  of  meteors,  209. 
Reflecting  telescopes,  282. 
Refracting  telescopes,  285. 
Refraction,  113. 

Relation    between    comets    and  me- 
teors, 214. 
Right  ascension,  42. 
Saros,  151. 
Saturn,  173. 

rings,  175. 

satellites,  177. 
Seasons,  92. 
Sextant,  296. 
Signs  of  the  ecliptic,  41. 
Solar  prominences,  49. 
Solar  system,  general  view,  29. 
Solstice,  40. 
Spectroscope,  302. 


INDEX. 


315 


Spectrum  analysis,  298.      "". 
Square  of  Pegasus,  233. 
Star-gauging,  273. 
Stars,  218. 

colors,  228. 

distances,  225. 

magnitudes,  220. 

motions,  227. 

names,  219. 

twinkling,  229. 

Statistics  of  sun  and  planets,  35. 
Structure  of  the  universe,  272. 
Sun,  44. 

chromosphere,  48. 

corona,  47. 

distance  and  size  of,  45. 

faculae,  59. 

heat  of,  61. 

light  of,  60. 

parallax  of,  44. 

past  and  future,  63. 

photosphere,  50. 

prominences,  49. 
Sun-spots,  51. 

how  to  observe,  55. 


Sun-spots,  periodicity  of,  57. 
Taurus,  232. 
Telescopes,  282. 
Tides,  !!&__ 
Time,  how  found,  105. 
Transit  instrument,  293. 
Twilight,  116. 
Twinkling,  229. 
Universe,  sidereal,  274. 
Uranus,  181. 
Ursa  Major,  230. 
Ursa  Minor,  231. 
Usefulness  of  astronomy,  2ft 
Variable  stars,  253. 

how  to  observe,  257. 
Venus,  70. 

transits  of,  71. 
Virgo,  234. 
Vulcan,  65. 
Week,  106. 
Year,  107. 
Zenith,  19. 
Zodiac,  43. 
Zodiacal  light,  213. 
Zones,  98. 


THE  END. 


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